There is an international standard as to which sections of the plugs provide what type and how much of a specific voltage. That is why there are so many sizes and shapes of all the different plugs (to meet all the different voltage requirements of various devices).
Generally speaking, if the plugs fits at all, then there is a pretty good chance that it will work, and there is only a VERY slight chance that it could actually cause any harm to your device.
However, to make sure that it will work for your device, you'll still have to look on the back of the adapter and check it's voltage output, and then compare that to the requirements of the device you want it to power.
You can also get inexpensive "universal" power adapters that come with lots of different plug fittings that you can use for all sorts of different devices (think Radio Shack, Best Buy, Walmart, etc.). Just try them all out and when you get to one that fits your device and works, you're in business.
That's pretty much all there is too it. Good luck!