What are the safeguards in the German constitution of 1949 against dictatorship?

I'm actually not sure about the premise of the question. The main safeguard isn't Constitutional in the first place, but simply the memory of people who experienced the dictatorships. As far as Constitutional guarantees go, the Grundgesetz is actually fairly weak compared with the US Constitution.

The main safeguard is a separation of powers, mostly modeled after the US Constitution (there is a President, there is a Bundestag similar to the House of Representatives, a Bundesrat similar to the Senate, and a judiciary similar to the US judiciary). There were some changes made compared with the US Constitution. Whether they strengthen or weaken the checks and balances is largely a matter of opinion.

As others have pointed out, a number of human rights have also been written into the Grundgesetz. I would argue that this is not a true safeguard, because the Grundgesetz is much easier amended than the US Constitution, and at least two of the original human rights were already abolished in the 1990s. Germany sees on average one change to the Constitution every year (60 so far since the GG was introduced), while the US only had 27 in the 225 or so years since the Constitution was passed.

Even without amendments, one core human right is much narrower in Germany than in the US Constitution. Germany does not have freedom of speech, but only freedom of the press.

Edit to clarify freedom of speech vs freedom of the press, as requested by @Bregalad

First of all, this depends on the definition of freedom of speech that you subscribe to. Many of the restrictions that are considered "normal" in Germany would be called entirely unacceptable in the USA (and, for that matter, in many other countries, too). People from the United States generally take the attitude "I may have utter contempt for what you are saying, but I will stand and fight to the death for your right to say it!" - and that includes even things such as Nazi political positions.

One problem with using arguments of the type "XXXX is a free country" is that there is no such thing. No country is a "free" country, just as much as no country is a completey "unfree" country - it is all a matter of degree. What is more, most people tend to judge based on their own experience, so if you ask an American and a German about freedom of speech, they, in a way, don't even speak the same language. I have the benefit of having lived in both countries for several decades each, which allows me to compare and see the drawbacks of both sides.

That degree-of issue includes freedom of speech, too. Even in the USA, there are restrictions on freedom of speech (libel comes to mind, as well as yelling "fire" in a crowded theater, or, controversially, obscene material). Of course Germany has some freedom of speech, just significantly less than the USA does.

Now on the details why Germany has significantly less freedom of speech:

The Grundgesetz in Article 5 specifies:

(1) Jeder hat das Recht, seine Meinung in Wort, Schrift und Bild frei zu äußern und zu verbreiten und sich aus allgemein zugänglichen Quellen ungehindert zu unterrichten. Die Pressefreiheit und die Freiheit der Berichterstattung durch Rundfunk und Film werden gewährleistet. Eine Zensur findet nicht statt.

Translation: "Everybody has the right to express and distribute his opinion in word, writing and images freely and to educate themselves from freely available sources without interference. Freedom of the press and freedom of reporting by radio and film are ensured. A censorship does not take place."

(2) Diese Rechte finden ihre Schranken in den Vorschriften der allgemeinen Gesetze, den gesetzlichen Bestimmungen zum Schutze der Jugend und in dem Recht der persönlichen Ehre.

Translation "These rights find boundaries in the rules of ordinary laws, legal regulations to protect youth, and in the right to personal honor".

(3) Kunst und Wissenschaft, Forschung und Lehre sind frei. Die Freiheit der Lehre entbindet nicht von der Treue zur Verfassung.

" Art and sciences, research and education are free. The freedom of education does not release from allegiance to the constitution."

For comparison, here is the US First Amendment: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances."

Differences between true freedom of speech as in the First Amendment of the US Constitution and the German version:

• The German version is limited only to words, writing and images, and only to personal opinions. In the USA, Freedom of speech includes every form of expression, including art, clothing, support for political candidates, etc., and factual reporting, fiction, etc.
• The German version includes a clause that explicitly allows restricting freedom of speech by ordinary laws. This is actually what makes German "freedom of speech" unsuitable as a safeguard against dictatorship. The US version does not allow ordinary laws to restrict freedom of speech (even though this is often attempted, such laws are routinely overturned).
• The German version prohibits censorship, but does not protect against prior restraint.
• One area where the German GG Art 5 does offer an additional protection is that it also guarantees access to material.

This has severe practical impacts as it is. There are books that are banned in Germany. The obvious ones are Mein Kampf and Das Kapital, but also a lot of adult material (even books that are considered high literature in other countries). Sometimes, there is a way around that: I have seen a few ostensibly illegal books put into the appendix of a (pseudo-)scientific essay about the book. That put the whole work under the protection of Article 5 as a scientific work.

Germans think nothing about prohibiting political parties that hold unpopular positions. Americans would be outraged if anybody demanded that the Nazi party or the Ku Klux Klan were outlawed - they rather deal with them in open discourse.

Certain topics are completely illegal to talk about in Germany. The best-known example is of course Holocaust Denial. As abhorrent as Holocaust Denial is, it is also a very blatant case of not having freedom of speech. Freedom of Speech is specifically the freedom to say unpopular things and let the democratic marketplace decide about ideas. Or, as Rosa Luxemburg said "Freiheit ist immer auch Freiheit des Andersdenkenden".

BTW, in the USA, too, insulting speech is illegal; it is called libel. One big difference between German and US law is that in the USA libel is only illegal if it is untruthful, not when it is merely an opinion. For instance, "Hillary Clinton and Carly Fiorina are sl*ts" would be legal in the USA (because it's an opinion) but "Hillary Clinton and Carly Fiorina are prostitutes" is an illegal statement of a false fact.

Now to be clear: freedom of speech as they practice it in the USA does have drawbacks, and one could make a case that some of the restrictions in Germany could be an improvement. I would disagree, but it would still be a valid argument.

But in the context of the question: the German version is unsuitable as a safeguard against dictatorship, because only some speech is protected, and also because ordinary laws can restrict it further.