If I don't see it on the actual print (usually at the end or the beginning), I try to find out through research. It works pretty well in most cases. The difficulty gets bigger, the older the film.

It doesn't serve any particular purpose. I just never understood why the date a film was released is used, instead of the date it was copyrighted (imo that means the film was finished as a complete work at that point). I think it's more important to know when a film was made than when it was seen by a wider audience.