Krajowy Rejestr Karny (National Penal Register) is the institution you need to turn to. You can download the form for inquiry about a person here: bip.ms.gov.pl/Data/Files/_public/bip/krk/formularz_krk_osoba.pdf
- you need to fill it in Polish and return it to a Biuro Informacyjne or Punkt Infromacyjny of KRK that affiliated with certain courts in Poland. You have to pay a fee of 50 PLN and I am not sure if they will send the query results abroad.
actually a new law implementation will take effect starting from May and you will be able to place internet inquiries to KRK - so why not wait for a while if you are based abroad - if you are based in Poland you just need to find your nearest Punkt Informacyjny Krajowego Rejestru Karnego.
however a convicted person can apply for erasing of their penal record after some time of the end of their punishment - in minor sentences (below three years of prison) the erasure can take place as soon as 5 years after the end of punishment (if the convicted applies for it) - the condition is not coming in conflict with the law in those 5 years