edit: actually, you can use a guitar amp to play a bass. it will lose alot of lows. alot of people think that doing this will damage the guitar amp. but nobody has ever explained the logic behind it.
the fender 59 bassman is actually a bass amp. but only guitarists use it. it happens to sound good on the guitar, and sounds like shit on the bass.
some bass cabs are full range, like the accugrooves. those are, well, full range. self explanatory.
if u are asking about how to physically IDENTIFY and differetiate a bass amp from a guitar amp, there's no way of simply looking at an amp and being able to tell. you'll need to recognise what model it is.