Yeah, just don't make a big deal out of it.
My girl sometimes (maybe once a month) goes to club with some of her girlfriends without me. I'm very cool about it, because I also go out without her.
But if you call her the next day, just don't act mad or insecure because she "could" have done something, just ask her how it's been and if she had fun.
The thing is, if you actually start to act all jealous and stuff, she will lose respect for you, and if that happens, she just can't love you anymore.
So... just play it cool. Go to Articles on the Sosuave homepage and read the article Are you the jealous type? by Allen Thompson. Trough that article I came on sosuave in 2000 (My girlfriend at the time dumped me, and I was searching on the net why people are jealous, and that's how I found Allen's article). And guess what, after 2000, I've never been dumped again, since then I've been always in control and I am the one now ending relationships (only if it's necessary ofcourse
).