Is it possible to find out if a Polish citizen owns a house?
Yes, but only if you know which house. And even if you find out which property he owns, there are ways and means for him to put that property in effect more or less beyond the reach of the bailiff (or at least to make it very unattractive to and time consuming for the bailiff, for example encumbering it with rights that make it difficult to sell or by subdividing it into different parcels or applying for it to be reclassified, etc).
This is for protection not malicious reasons before someone has a go.
If you're looking to protect yourself, shoot me a PM with the details and I might be able to suggest some tried and tested techniques.