Look at the Right side of the rear sight.
It should be marked with one of the following:
––––––-
––––––- two lines like an equal sign is very tall
––––
–––––––– short line above long line is the tall
–––––––– a long line is the normal sight and fairly standard
––––––––
–––– short line under a long ine is the short sight
These are also the four exact position heights of the Glock Adjustable Rear Sight.
The plastic sights are a couple of dollars and easily changed if you can borrow a Glock Sight Tool. Once changed and windage zeroeed, they never change.
As a practical matter, almost all M23s are sighted about 3-4" above the point of aim at 25 yards. If you are shooting low, consider that it might be a consistent trigger yank. I personally and me have never seen a Glock shoot low and absolutely not a M23. The only way an unmessed with factory standard gun would be low is by shooting 85-90 grain 9mm ammo in something like a M19. The problem with them is that they actually have about a 75 yard zero with standard ammo for the caliber and always shoot high at 25 and 50 yards unless you yank them down with a poor trigger pull.
Which is why I have a Glock tool and put the correct sight in for a point of aim/point of impact at 25 yards setting.