How can you tell which rail on your gun you have?
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.