Look on the back. If there's anything extra than just the Disney logo, hold on to it and check around. If you see "Cast Member Exclusive" or "Lanyard Pin series" or "Limited Edition (number)", then just ask around.
You can also go to PinPics.com and search for the pin. It will show you how many people have it on thier "wants" list or "have' list or "tradable" list. This will give you a good idea on how many people want it and thus the value.
A dollar amount is difficult to measure a pins value on. Some pins aren't even for sale (although others try to sell them once they have them).
Every pin has its own value for different reasons. I would read on the boards alot (trade and "for sale" boards) before trading that first pin if you're REALLY worried about it.
There's a rule about treating newbies very nice and not to take advantage of them. If someone can knowingly rip you off without remorse over a pin.... well, let's just say, "What goes around, comes around!"
When in doubt, just ask several people. Hopefully, you'll find consistant answers.
Have fun trading!
PinCollector