Four dots after the 12th fret "double dot" inlay, with NO "extra" fret after that fourth dot inlay = 22.5" scale neck
Four dots after the 12th fret "double dot" inlay, WITH one "extra" fret after that fourth dot inlay = 24" scale neck.
Also, you can tell from the neck codes stamped on the necks. If the code starts with a "3/4" or with a "9", it's a 22.5" scale neck. If it starts with a 8 or a 16 or a 49, then it is a 24" scale neck. That first digit (or two) is a model code, not the day of the month!
The letter at the end of the code indicates the neck width at the nut. A=1 1/2" B=1 5/8" C=1 3/4" D=1 7/8"
Example:
8DEC65B
8 = 24" scale Mustang neck
DEC = Month (December)
65 = year (1965)
B = 1 5/8" neck width at the nut - which was the "standard" Fender width, although A width necks are fairly common on shortscales too - especially on the 22.5" scale necks.
I actually have a nearly pristine 1965 Mustang neck with that exact code on it (8DEC65B). I bought it for my '75 Musicmaster / Duo Sonic project, but only had it on the guitar for about five minutes before I pulled it off - it is a bit "thicker" (not quite 1", front to back; .9" IIRC) neck with more of a U shaped, as opposed to a C shaped profile to it, which doesn't sit as well with my short / stubby fingers. I'll probably eventually put it back up for sale for what I have into it - which wasn't cheap (~$425), but it's the prettiest looking slab board '65 neck (complete with original white button F tuners) I have ever seen. I just wish it would have fit my hand better.
![Sad :(](./images/smilies/icon_sad.gif)
But in the end, I found a '94 MIJ Mustang neck that fits me like a glove, so it all worked out. It's not a vintage USA slab board, and it isn't in the same great cosmetic condition, but it works with my hands, which is, IMO, far more important.