That's a good question, Sahara! I can only tell you what I did, which was to assume that any oversized drinks, etc., in the passenger's possession were purchased airside.

I'm not sure what the official policy is -- I completed an online instruction module on gate screening about 6 months ago, but wasn't actually tapped to do it until recently, whereupon I couldn't remember the protocol. I informed my supervisor, and he told me that my partner understood it, which (as it turned out) was not the case. He was clueless, too! So (as so many here accuse the TSA of doing), we simply made it up as we went along!