I got a bunch of wine from an estate sell.. some of them are 30 years old. How do I find if they are "worth" anything.

I don't plan on "selling" them but I do expect to take a bottle to certain social events and so on. I wouldn't want to bring an expensive ass bottle to an event that wouldn't warrant it and give people the wrong idea. Also I wouldn't want to bring a cheapo bottle to something that should have something nicer.

Whats a good resource to use? I would imagine just because something is old doesn't mean it is expensive right?