Cut a sample and put it in a bucket of water. Let it sit there for days and see if there is any sign of delamination.
Look on the internet and try to find info about a boiling water test. You take a small sample of the plywood and boil it (not sure how long). See if the hot water degrades the glue. Exterior glue should hold up.
Examine the plywood around the edges and try to get a feel for how many voids it has in the interior ply(s). Supposedly one of the things that makes marine plywood special is that it has very few if any voids, whereas BC pine or common luan, for instance, is allowed to have quite a few voids in the core veneer(s).
Does it bend well - consistently, that is?
How is the surface quality? Nice and smooth? Not many patches? Good looking grain? Any sign of bubbles in the surface veneer, like they maybe failed to use enough glue?
One comment I'd make is that using mystery plywood that was used for packaging overseas is obviously going to be a gamble. When you consider that a lot of us use pretty cheap wood, ($10-20 dollars per sheet), the wood isn't really that big a factor to start with. You'll have way more money in epoxy and glass than you will in plywood.
Lots of guys use common luan underlayment (about $10 per sheet). I've built a couple of boats with BC pine (about $15 per sheet last time I looked). For the pirogue I'm building right now, I used "Ultraply XL", which I found at Lowe's, being sold as "premium underlayment". The stuff I was $20 per sheet, had excellent surfaces on both sides, the "A" side was a pretty mahogany type wood, and there were very few voids. Plus the manufacturer has a website and they confirm that it is made with exterior glue (you can't always confirm that with luan; can't always figure out who even made it, much less what kinda glue they used).
So my point is, why gamble on mystery wood when you can get pretty well identified product for $10-$20 per sheet -- call it $40-$60 per boat? You will have over twice that much money in fiberglass and epoxy, so the savings just doesn't seem worth the risk.
I fully understand being on a tight budget... that's why I'm not using marine mahogany. Just wanted to give you something to think about.
George