Instead, I would say "I can/could check if your Polish has improved". Even that sounds strange, because it's hard to imagine a scenario where I (as a Polish speaker) would ever say such a thing (presumably to a non-speaker). For example, as the non-speaker, X might say "I dunno if I'm ever learn this language. I try talking Polish to people, and they switch to English". My response should be (in some form) an invitation to X to speak some Polish, and we'll try to carry out a conversation in Polish. So I might say "Do you want to try talking Polish?" (well, more likely "Porozmawiajmy!"). The point is that if you're interested in sounding natural, your first concern has to be the message. Expressing an interest in whether a person has improved their language skills, and especially testing them on that, would be low on the list of messages that I'd ever want to send.
I suppose if I were the language-evaluator for a job that requires knowledge of Polish and I had to see if you were up to the required standard, I might maybe say something like "(In the course of the chat) I'd check if your Polish has improved". Even then, people would say "I need to check". Saying "I'd check if your Polish has improved" almost sounds like a threat.
