300 hands is maybe a decent sample to see how tight/loose someone is but it is completely irrelevant to determining whether they are a winning player.

In determining whether someone is a winner over the long run, I believe I read a post once where someone said 1K hands is not even close to a reliable sample, 10K hands is more of a dent, 20K is getting there, and you should have a pretty damn good idea whether or not your winning after 30K. By that time variance has evened out for the most part. Just remember people can go on swings in a few hundred hands where they are up 'x' number of BI's. I've seen fish running 43/2/.4 over a few hundred hands and yet they are up $15 at 5NL.

I myself don't rely on how much money someone has won to determine whether they are a 'winning player'. Just focus on how they play. It's probably self destructive if your worried about someone elses product and not trying to figure out their process, just as its important to focus on process as opposed to product within your own game. (if you need me to sort of elaborate just reply..its really late here I'm half asleep)