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If you're referring to the distinction between a MIL-STD-1913 Picatinny Rail and a Weaver Rail, your answer is to use a pair of calipers. Weaver Rails have a slot width of .18 inches, with no standards regarding the spacing of slot centres. Picatinny rails have a slot width of .206 inches, with slot centres spaced at .394 inches.
General rule of thumb is that, if it's intended for commercial, hunting or other type of sporting uses, it will usually have a Weaver rail, while military styled and tactical designs will have a Picatinny system (to include rifles dressed up as sporting guns, such as the 'Remington' AR15 rifles currently sold at places such as Dick's Sporting Goods).
However, Picatinny rail systems are available for commercial sporting firearms and vice versa.
In the case of Airsoft weapons, although they try to emulate designs with Picatinny rails, the measurements are probably Weaver, or closer to it, as most airsoft accessories are designed to be Weaver compatible.
