How do you know you are about to get fired?

Never.

It's one of the things that can happen anytime, though in Germany very strict laws make it impossible to fire on a whim.

I'm Germany, you quite literally have to steal, sell inside data or behave in an extreme way like pooping on your managers desk with a greeting card making an explicit statement to get fired.

Simple answer, you don't.

You can certainly speculate though.

Have you:

- recently had a poor performance review
- been pulled off of a major project or two
- pissed off someone in leadership

What you want is to confirm your suspicions.

- request vacation time 2-3 weeks out
- ask to work on a major project
- ask your supervisor directly if you should be concerned about layoffs in your company

That last one really only works if you're face to face. Body language and vocal cues are important to determine truth, because no manager will be honest about it.

If you're concerned about it, start looking now.

Your boss avoids eye contact and asks around what you do behind your back.

Missing the strongest indicator of them all. Is the company seriously struggling?

Some indicators for that is a complete lack of innovation for years or a serious commitment that don't pan out (like starting in a new country).

This is very much dependent on location. Like @IntrusionCM said, in Germany it's practically impossible to get fired. UK is similar, although you can be made redundant at any moment (which is definitely not the same as being fired), but there are still strict rules around this.

However, in a lot of states in the USA, you can simply be told not to bother coming back to work, for absolutely no reason.

Your hand is moving towards the managers face.

When you get a random meeting request with hr and your manager

What did you do?