Upvote:7
As nobody knows whether your name really is Franck and the account picture shows your own face, there is no way to prove this on Travel.SE.
More generally speaking:
Get your passport stamped upon entry
This obviously works, but depending on where you are coming from and into which country you travel, there will be no stamps (or even border controls).
Take a selfie in front of a landmark
Show the bill (with your name) of a hotel
Get fined (or even jailed) and have a report of it with your name on it.
Get yourself into a local newspaper, TV station or similar
Speak at a well-known (and documented/published/streamed) event in the country
Obviously, any "proof" based on data/pictures you provide yourself can easily be faked with Photoshop or other means. So an independent validation of your presence (as in the last two points above) is prefered.
Upvote:7
How can one prove one has entered a country?
You will have to ask whoever wants that proof (for whatever purpose)
If an entity requires "proof" they typically will also define exactly what types of proof are acceptable and which are not. That will vary greatly from one situation to another. That's especially the case if "proof" is needed for some legal reasons.
A geo tag on your phone or a hotel bill may be good for friends and family but maybe not for a criminal investigator.