When most people say marine ply they mean what was WBP ie plywood for exterior use, not for building boats (which has a 300% or so mark-up but a more stringent certification standard).
If it is clean and new enough, you should find markings on the edge stating its certification. If it is relatively new there should be two markings starting with EN, EN636 is the wood it is constructed from, EN314 is the glue. Each should then have a -suffix of 1, 2 or 3. The 1 suffix means interior only, the 2 means more tolerant of humidity/damp and the 3 means for exterior usage. These EU standards are a bit more sophisticated and useful than the rather naive BS for WBP which concentrated only on the glue-line and ignored the fact the plies could be made of something like birch (or cork-bark). -3 is the WBP (or in the colloquial 'marine') equivalent.
If its old and raggedy you probably won't be able to tell from markings, but if the glueline is a dark red-brown it will likely be phenol/resorcinol which is the usual WBP adhesive. As to plies, you are on your own but being a tropical brown colour and tight and hard grain is good, even better if makes you sneeze when you cut it (on the probably rubbish theory that bad for you is bad for all organisms).
