Flat rather than mushroomed drive flange ends (hub end :) ) on the halfshaft mean 24 spline.
Sorry to disagree Andy, but my 10 spline ones have flat drive flanges....... they changed from domed to flat when they went from imperial to metric......
i'm not aware of any dead sure way to tell, apart from pulling a shaft out, which for the sake of 5 bolts and a gasket is so easy you might aswell do it to be sure!!