I am in college and our group is mostly mixed between 21 year olds and 20 year olds. I know that it would be illegal to have minors drinking in my apartment, but what sort of liability could I have if I am at a party where the host is allowing minors to drink? This is assuming that I didn't not share my alcohol or buy alcohol for any of the minors. I do my best to avoid situations like this because I don't want to be involved with the police at all, but unfortunately, with the way parties work in larger places, there could be underage people let in and given alcohol without me even knowing it.
If you want to be 100%, don't attend those kind of parties.
On a more realistic note, you have nothing to worry about as long as you aren't the one hosting the party, providing the alcohol, or contributing to the delinquency of a minor in any other way.