Imply your question jokingly. That’s the easy route. When she mentions something about the party, say, “Yeah…well I guess you’ll have to tell me about it since I wasn’t invited.” Just make sure to say this jokingly with a big smile on your face. She’s bound to respond one way or the other. Or, as mentioned, be up front and ask (this is the best option, most honest) – if you’re afraid of offending her (which, IMO, you shouldn’t be) at least you have Option 2.