These three forms of radiation have different penetrating properties. To tell them apart one puts various kinds of absorbers between the source of the radiation and the detector. Depending on how much and what type of material they can pass through you can get a pretty good idea of the type of radiation.
1. Alpha rays are actually ionized Helium nuclei arising from spontaneous or neutron induced fission of heavy elements. They have very little penetrating power and can be stopped by a sheet or two of paper. By the way Ernest Rutherford, the Nobel Prize winning physicist who used alpha particles to discover the properties of the atomic nucleus, was a student at Canterbury College in Christchurch. There is a little museum there you can visit showing some of his early work. Worth a visit.
2. Beta rays are either electrons or positrons arising from nuclear beta decay. They can pass easily through paper but can be stopped by a few millimeters of Aluminum.
for some more information.
3. Gamma rays are a form of electromagnetic radiation, just like radio waves or light or x-rays, except they are far more energetic. Depending on their energy they can pass through several millimeters or even centimeters of Aluminum. Lead is a more effective shield.
There is a nice article in Wikipedia on radiation and associated shielding:http://en.wikipedia.org/wiki/Radiation_protection
(published on 08/15/2009)