Darren is right on the money!
If you did need a fret calculator though you can use this one
which will save you time and trouble.
If you simply want to know what the scale is of a particular guitar is without doing any math, just measure the distance from where the string leave's the nut to where it touches the bridge.
In most case's your only going to find the following scales.
25.5" scale found on almost every modern day bolt on strat style
24.75" scale found mostly on Gibson necks.
25.4" scale found mostly on Martin acoustics.
25" scale found mostly on PRS