My wife got a set of old polish coins from her grand mother from 1960/70s.
We don't have an intention to sell them but would like to know their value as of today, so that we can preserve the most valuable coins carefully.
Is there any way to know the value of these coins?
The value of any antique or vintage item varies depending on which market you're selling it in. Basically you have two markets for Polish coin collectors, Poland and the Polish community wherever they're located in the world. I imagine that a lot of people of Polish origin, like Polish Americans who are serious collectors would buy not only from American coin dealers, but directly from Poland. This Polish site might be useful to you in showing how much your coins might fetch on the Polish market:
Also there's a big coin fair coming up in Warsaw a couple of weeks from now so keep an eye on this site (it's in English).
As far as the American market goes check out a few American coin dealers.
Also bear in mind that the values you see obviously include the dealer's mark up so he would expect to buy them for less than that from you. If there are any rare ones in the collection and you did ever decide to sell them your best bet is probably to put them in a specialist auction.