Scale length has to do with how far the distance is between the nut and the bridge. It has nothing to do with how many frets the neck has on it. Say the scale length is 25.5" that means the distance between the nut and the bridge is roughly 25.5 inches REGARDLESS of how many frets are on the neck 22, 21, or 24 frets, it doesn't matter. the bridge will still be 25.5 inches from the nut. A 24.75 scale neck has the bridge 24.75 inches from the nut... every time . The difference between the two scales is the distance **between **the frets.

So you need to measure the distance between frets and determine if it a 25.5 or a 24.75 inch scale neck and then make sure the bridge sits the appropriate distance from the nut. Most just divide the scale length by half and make sure the 12th fret is that distance from the bridge. It's just easier to measure 12.375 inches (half the 24.75) than it is the 24.75 inches accurately. If you don't have a micrometer the easiest way to do it is with a tape measure. Measure from where the strings leave the nut to the center of the 18th fret. If it's pretty much 16" dead on you have a 24.75 inch scale neck. If it's closer to 16 1/2 inches it's a 25.5 inch scale neck. ( technically 16.484 inches but 16 and close to a half will work to determine the scale length)

I am sure I made that clear as mud but at least i tried!

Be sure when you are measuring things it where the string leaves the nut or the string rests on the saddles of the bridge