How do custom officers determine if the imported goods of a passenger are old or newly bought?

To add the other answers (the one by Relaxed is spot-on – customs officers use their intuition), here comes some information about the legal situation, adapted to your case (Germany). Note that IANAL – so this is information is all only to the best of my knowledge.

Legally, it is enough for the German Customs officers to suspect that something is imported to the country for the first time. There is no legal necessity for them to prove it before they can ask you to pay customs (and a fine for undeclared goods). Of course, in obvious cases of wrong decisions by them, your chances to appeal are quite good. But in principle, it is the reponsibility of the traveler to provide a proof that the goods you bring into the country have been brought back by you. This is particularly relevant in case of electronics.

There are two ways to do so – either you carry a receipt for the good with you that lists a serial number and that thus proves that the good has been bought within the EU, or (2) before you depart from Germany, you go to your local customs office with the goods and have them prepare a form for “good re-entry” for you. The official term for this is “Nämlichkeitsbescheinigung” (which is probably the most uninformative term ever invented – it roughly translates to “Proof of Particularity”, which means nothing – just as the German term). Typically, your goods will need to have a serial number for that approach to work.

There are several tell-tale signs (condition, packaging, receipts, having several identical items, having things you would not typically need on a trip abroad) that might suggest an item has been bought during a trip. Beyond that, I guess customs officers follow (unpublished) guidelines or use their own judgment but they do in any case have a lot of discretion. Even where you presently come from might matter (I don’t have any strong evidence for this but I have the feeling that flights from, e.g., Hong Kong receive more scrutiny than others at my local airport – even if it would be trivial to walk to another exit and mix with the flow of travellers from other EU countries).

One thing I did hear once from a friend who is a customs officer is that they rely a lot on people’s dumbness, ignorance and on informants or tips to catch smugglers (small and big). There are enough people carrying brand-new electronics complete with packaging and receipts from the place they come from to make it worth their while. But whatever they do is aimed at providing some deterrence and recovering money, nobody expects it to be a 100% accurate process and people do manage to import things illegally everyday.

The most important thing to note is that your thinking seems predicated on the notion that customs officers need to prove you bought something abroad. It isn’t necessarily that way, at least not to collect duty (as opposed to bringing criminal charges). I don’t know relevant German law at all, but a quick Internet search suggests it’s like that in Germany, too.

How you are supposed to prove something is not new is indeed by producing some documentation establishing when and where you bought the item. From your perspective, not carrying receipts for your things is a given but from the authorities’ perspective, it isn’t so. So the fact that people typically do not do it isn’t a problem for them as much as it is for the people concerned. If an agent determines something is new, you might not have much effective recourse if you cannot prove otherwise (if you still have it, you might still produce the receipt afterwards even if you didn’t carry it with you, though).

If your equipment (e.g. camera) is professional grade and particularly expensive, you might even want to go one step further and secure temporary export paperwork to ensure an easy return. Some countries also discourage traveling with a lot of jewellery for that reason.

In some cases the custom officers can see if something is bought on your trip or brought by you from home, by looking at the serial number. Quite some electronic devises keep a log of serial numbers, country where it was sold, etc.

However in most cases, it all is very subjective and probably dependent on personal decision by custom officers. The only thing, in my opinion, you can do to prevent this is to keep proof of purchase of the goods.

I do this by storing scans in dropbox and always have access to my dropbox from my devices (tablet/smartphone). Since you will probably not bring that many brand new devices you could also choose to simply carry a copy of your receipts with you.

I don’t think you will find a concise answer and even if there was one, Customs & Immigration Departments would not want it known publicly. In very generic terms … They Observe. They are trained to read your body language, to interpret the way you answer, to go beyond the surface. They look at how you dress, what you have, where you were, how long you were there, why you were there. And then they make judgement calls based on what they have observed.

While they have set examples they use in training, it is mostly time on the job that improves their judgement. Perhaps the customs agent you had a hard time convincing was new on the job or maybe was just having an off day.