Is it possible to find out if a pension check was cashed about 25 years ago?

Yes. Banks have to maintain records of all checks that were cashed by them. So if you want information from 25 years ago, banks may not be able to help you readily because the data is from so long ago. But, you can get a court order asking for this information if it is important and in that case the banks have to dig deep into their records and will give you the information you want.