I believe you can email Disney with pics of the tix and they'll tell you if they're valid. I seem to recall doing this a few years ago when I found a one-day ticket that I wasn't sure if I'd yet used. They got back to me relatively quickly.
Remember that they take pictures and connect them to the tickets, so it's essentially impossible to "rent" tickets from shady characters now. With Disneyland tickets, generally, if it sounds too good to be true, it is. There are trustworthy discount sites (lmtclub, ARM, Park Savers, and others)... but the savings are generally modest (less than $50 savings per person).
You can't...which is why unless you buy them first hand from Disney directly or a verified legitimate reseller you are susceptible to many of the rampant ticket scams with zero recourse. It is not Disney's problem if you buy from a third party and get taken.
Happens every day. Not worth trying to save a buck.