otherwise jack up one front wheel and turn the wheel and see if the front prop shaft (drive shaft) is turning. if turning engage 4x4 and see if you can still turn the front wheel. (assuming you havn't fitted a front locker)
it won't spin if prop shaft is spinning due to the diff.
if 4x4 engaged and prop shaft is not spinning and you can spin one wheel while the other is not spinning then you have a broken diff.
or a faulty hub - not engaging properly. There's actually not really much meat that they engage with. If you installed the hubs yourself I would remove check and refit the sus one.