How could I tell if I was drunk?

Very complex question to answer, but you were probably borderline "drunk" (intoxicated). A big fellow like you (I'm your same height -- 6 ft., 2 in. -- but bigger/heavier at around 100 kilos) can probably metabolize one beer/glass of wine/drink per hour. Depending on what was in the punch, it sounds as though you were bumping right up against that level. The only dependable measurement of intoxication is Blood Alcohol Content ("BAC"); anytime it gets close to 1/10th of 1% (0.1%), you're VERY far gone. In the absence of a blood test, one good clue would be to ask yourself if you did/said anything among your friends that you might ordinarily not have done. Made some grandiose claim regarding your career, intelligence, athletic prowess, etc.; hit on some friend's girlfriend/wife; got into an argument with someone over politics/religion, etc.