It's nice that some handguns are marked as +P capable, but doesn't seem to be common practice. Read the manual to see what mfr says about +P ammunition. Manuals are available free for download, either from mfr's website or online repositories.

Every pistol I've owned has been +P capable. None were marked to that effect--Colt, S&W, Ruger, Walther, AMT, Glock--but the manuals gave the information.

I did own a Diamondback DB9, which had no markings prohibiting +P--but the manual explicitly stated the gun was not designed to handle +P. I know someone who's DB9 'violently deconstructed' from one round of +P.