Obviously, if the spying was done well, we wouldn't have known.
The word would be passed around: "watch him, he seems to be from 'spółdzielnia ucho' (The Ear Cooperative)'". They operated also in academic environment, but often in a clumsy way: always at night time in best restaurants, surrounded by "cinkciarze" (cincz many, cieńć many, change money), easy girls and prostitutes, spending money on expensive drinks for themselves, their victims, or potential collaborants. Dormitories actually were not good environments for the informers, because of intimacy of living together. Little details were the tale tellers. Tourist groups (I was a part of one such informal band) were not their hunting grounds either; they were just too lazy for the extensive efforts as it was much easier to find their prey in town.
We had one informer in our class; he was quickly discovered and visibly ostracized. He dropped from the academic courses, has become a visible alcoholic, and then vanished somewhere. I did not know more than two among students in our Math-Phys-Chem faculty, by there were many so-called "eternal students" of Law, and some of them had a reputation of being the informers.
I knew many people studying "zero-perspective" course of studies: archeology, ethnography, history of art. Some of them could not get any job after graduation and decided to work for the militia (this is how police was called then), as experts in various fields. They probably never spied on anybody, but there was a shadow of the doubt following them around.
One such archeologist worked for the Voivod's Office, running the Department of Religion, or whatever it was called. They kept track of all black sheep among the clergy in order to blackmail them and use them for propaganda purposes. That worked both ways: the dirty boys would get apartment for their girlfriends or boyfriends, etc. So he was kind of spy, though he did it openly.
Yes, those were strange times to live. But definitely not as bad as in DDR.