Upvote:2
Within the Schengen area there are no systematic border controls. You can still be asked to show your passport, but all the authorities will be interested in that your passport is real, that it is yours, and that you have a valid visa or residence permit. In practice the 90/180 day rule is not enforced at intra Schengen borders. There is no practical way to do so, as your travels around Europe are not monitored. (And this thankfully so).
However it may be enforced at your place of residence. Most European countries require you to officially take up residence, and register with the local authorities at the place where you spend most of your time. This is a concept that people from common law countries may not be familiar with. And they can require you to prove that indeed you official residence is where you spend most of your time.
I had to do this when I moved to my present residence in Switzerland (and I am a Swiss citizen!). I travel a lot, spend a lot of time abroad for work. I had to prove to the local authorities of the village that I wanted to make my primary residence, that indeed, this is where I spend most of my time. I did that by giving them a printout of all the train tickets I bought in a year, showing me going away (and back) to this place on a regular basis. (That I buy all my train tickets on line made this easier).
So if you are a German D-permit holder, and have registered as a resident in, for example Berlin, and the authorities find out that you have not spend enough time at your registered address to qualify as a resident then you could indeed get in to trouble. So in this way the 90/180 rule, which really is a "spend most of your time at your primary residence" rule is indeed sort of enforced.
Upvote:3
how much proof would I need to provide to be given clearance?
There's no fixed answer to this question. It depends on your credibility and on the basis for the demand for proof of your presence in Schengen countries other than Germany.
when is this check likely to occur?
It is likely to occur if you come to the attention of police or immigration authorities in a Schengen country other than Germany under circumstances that suggest that you are residing there or at least regularly spending more than half of your time there.
When I fly home to my country after my residence permit expires, are they likely to pull me up at the airport and look over how many days I spent outside of Germany in other Schengen countries?
No. In that circumstance they are more likely to be concerned with the fact that you were present in the Schengen area after the expiration of your residence permit.
Or is it only gonna happen if I get unlucky at a border crossing?
It's almost certainly not going to happen at a border crossing unless you are wanted for a crime and there is a record of that in one of the databases that are checked when you cross the border.