Test grounding of wall sockets
These simple testers are easy to use and test more things than just the ground. Here's one on Amazon.  Here's the Amazon search I used. They cost about $10usd, often less. 

You just plug it in to each outlet and read the 3 lights. 

Convert USD to EUR here.


  So far, I could not figure out in what way my apartment house is (or: should be) grounded (no answer from landlord, I think they do not know themselves). 


Older houses may not have a ground at all. My house was built in the 1950s. It has a ground from the main electrical box to the outside, but most of the wiring in the actual house does not have a ground. I specifically had to run a separate circuit for my computer which had a ground, because a surge protector (part of my UPS) requires a ground to work.


  Is it then necessary to disconect the earthing pole from the rest of the house as I read somewhere?


Why would you do that? What is the purpose of this particular question? Did you hear you had to disconnect the ground pole from the main electrical box to test ground in the outlets?