A general copyright question

Is there a reliable way to learn if the printed version of the song I am looking at on the web is copyrighted. What prompts this is that I see instances on some of the other instrument websites/forums (not dulcimer) with tab or songs posted which are new enough I suspect they are copyrighted and should not be posted for all to use.

I realize many older public domain songs are indeed public domain, but if someone has written a new arrangement of an older song and copyrighted the new arrangement, is there a way to tell and know which is the public domain version? 

Or is it a case by case research challenge to make sure the song is indeed public domain? I don't record but envision a day when I may play at nursing homes, churches or senior centers.