How should I address a PI at the holiday party tonight?

Address her as "Dr." unless she invites you to call her by her first name.

The person best suited to answer this question would be your PI. You could simply say, "Hey, I'm so glad you could make it! By the way, do you prefer Dr. Samsonite or something else?"

It's likely that your PI doesn't know the culture of your particular lab, so she may even ask what people there prefer.

I would choose the first applicable option from the following list:

1. The way she has asked you to address her
2. The way she introduced herself to you in person
3. The way someone else from the lab introduced her to you in person
4. If you have a significant history of email communication specifically directed at each other (i.e. she is emailing you, not just emailing the group), the way she signs emails specifically directed at you, assuming it's actually a name or title and not, say, initials
5. The highest or most specific title (Professor, Doctor, or so on) you know or believe her to hold
6. If you really have no clue about #5, the default way you address other academics of similar standing who you don't know - probably either Professor or Doctor, depending on field

When meeting someone in person for the first time, items 1-4 tend to not apply and you wind up at #5, which basically reduces to Nicole Hamilton's answer.

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Note that generic forms of address like "Mrs."/"Ms."/"Mr." don't appear anywhere in this list. They would come in as a last-ditch default option if you weren't talking to an academic, but within academia, titles like "Prof." and "Dr." are common enough that you take them as a default.