Throw a party. And use a system like evite.com to handle the invitations. There is a typical pattern to the responses over time. You will have an initial flurry of yeses and regrets followed by a long period of silence punctuated by sporadic responses which continues to the days before the party. Then there is a final flurry and that is when you learn if your friends are real friends.
Because people come to your party for one of two reasons. Either they like you or they just feel obligated for reasons like you are an important co-worker or they don’t want to hurt your feelings, etc. Think of how these two types of people will handle your invitation.
An invitation is an option that can be exercised at any time before the date of the party. The people who did not respond immediately are waiting to decide whether to exercise the option. If she’s a true friend then this is because she has a potential conflict that would prevent her attending. She is waiting and hoping to avoid that conflict. When she is sure there is no conflict she will say yes.
The other people are hoping for an excuse not to come. Once they get a better offer, manage to schedule a conflicting business trip, or otherwise commit themselves, they will send their regrets.
In both cases, when the party is imminent, the option value of waiting is gone. Those who want to come but haven’t gotten out of their conflict give up and send their regrets. Those who hoped to get out of it but failed to come up with a believable excuse give up and accept.
So, a simple measure of how much your friends like you is the proportion of acceptances that arrive in the final days. Lots of acceptances means you better set aside a few extra drinks for yourself.