IIRC, 28 spline axles have a sort of hourglass-looking dish cut out of the hub, whereas the 31 spline axles have 3 holes drilled in the end. Or you could just count the splines.
-Jason
'72 Mach 1 || '90 Talon TSi AWD
http://nosaj122081.tripod.com/parade3.jpg
"Thats not a leak, my car's marking its territory!"
"If you've done it, it ain't braggin'." -Roy Rogers