Check these things, in this order.
Remove the magazine, and pull the bolt back with your finger off the trigger. Be sure it's unloaded.
If the bolt stays back when you pulled it back, it's an "open bolt" gun. If the bolt returns forward, it's a "closed bolt" gun.
Now, if the gun is an "open bolt" gun, this is how you check: cock the gun by pulling the bolt back. Be sure it's unloaded and with the magazine removed. Pull the trigger, to drop the bolt, and keep the trigger pulled - do not release it. With the trigger still pulled, pull back the bolt again. It should "click" into place and stay open if it is a semiautomatic. If, however, the bolt returns forward while the trigger is still pulled, it is full automatic (a machine gun) and should be disposed of.
If the gun is a "closed bolt" gun, this is how you check: cock the gun by pulling the bolt back and releasing it, so that it returns forward (be sure it's unloaded and with the magazine removed). Pull the trigger, and you should hear the hammer drop - but keep the trigger pulled. With the trigger still pulled, cock the gun again by pulling back the bolt. Let the bolt go slowly forward, and listen for a click like the hammer dropping (you shouldn't hear one). Let go of the trigger, and you may hear a smaller click. Pull the trigger again - if you do not hear the hammer drop, it is a machine gun.
These tests are not 100% conclusive, but they should be a pretty good indication.