In E”Y, after talking with many avreichim and after moving into many apartments that have had previous kollel families living in them, and myself trying to be careful about mezuzos, I have more than once found that the people before you are “relying on the people before them.” And, many times, it traces back to someone who wasn’t the biggest yarei shomayim, and, at times, even someone who wasn’t really frum! One guy told me, “we don’t even use those doors!” and, lo and behold, the mezuzos were pasul. I found a mezuza was even put in UPSIDE DOWN in an apartment: TWICE. So many b’nei Torah don’t even think about it – and they after all told me excuses.
Then, ESPECIALLY after talking with sofer friends, about checking their friends mezuzos in rentals in E”Y, DO NOT trust what came before you. ALWAYS check. Besides the fact you need to check twice in seven years. Call your local gemach to borrow to go and check. If you don’t have one, make one. If you can’t make one, take some maaser and donate to one.