You are welcome. I am glad to see that you spoke to an attorney on the matter. Regardless of the outcome, you should always know your rights and at least get an opinion on what happened.
As to your question: "Observation, by actually seeing her drink the can of beer? Or, observation by assumption/association? She had her one beer at 09:00 pm. The State Police did not arrive until midnight (when the bust occured). She was held until 4:00 AM, at which time a State Police officer wrote her on-view arrest citation. The kids say there were undercover ACB police at the party. (1)How would the undercover officer be able to identify her (specifically as consuming 1 can of beer, at a specific time) out of 142+ kids ? (2) If the ACB officer was able to generally identify her as just being there, is it legal for a different officer to cite her than the one who observed her? "
Observation, especially in the case of a minor consuming alcohol charge, would mean either that someone either saw her actually consuming the alcohol, or, they observed things that would lead them to conclude that she had consumed alcohol. This could include observations about her and her behavior: odor of an alcoholic beverage on her breath, other signs that would suggest intoxication (blood-shot/watery eyes, slurred speech, being unstable while standing, etc.). For a minor consuming charge, the police would not have to prove intoxication because just consuming is enough, but if they have evidence of intoxication, that makes the case that much easier to prove.
It would be the totality of what they testified that they observed that would determine whether there was enough evidence to say that she had consumed an alcoholic beverage. If the testimony was strong on that point, either because someone directly saw her drinking or the observations that suggested it, it is harder to refute.
If that were the case, then another thing to look at is if the police were in a place where they had a legal right to be when they made those observations. If they did not have a legal right to be there, then that would be a point from which the attorney could argue that anything they observed could not be used. There are a lot of circumstances that could bring that point up, but often times, the police are there legally. It really just depends on the facts of the case.
As to this part of the question: "The kids say there were undercover ACB police at the party. (1)How would the undercover officer be able to identify her (specifically as consuming 1 can of beer, at a specific time) out of 142+ kids ? (2) If the ACB officer was able to generally identify her as just being there, is it legal for a different officer to cite her than the one who observed her?"
The undercover officer may very well be able to identify her and then again, he might not. It really just depends on what testimony he gives. Because the party involved 142 kids, the defense attorney will probably hit this point home. Unless he knew many of the kids by face/name, that would definitely be something to argue, especially if he gave citations for the majority of the kids there. And, yes, it is legal for another officer to cite her. The Prosecution would probably just call both officers to testify about what they saw, did and what they were told.
Good luck to you.
