How can I tell a grass carp from a native carp?

I'm curious, are there any clearly definable traits that distinguish a grass carp from other carp species?

Occassionally, I'll catch a carp in a local lake that was stocked with some grass carp a few years back. Not knowing one carp from another, my 1st inclantation is to thow the fish out (treat it as an unwanted trash fish) but I'd hate to eliminate a desired species so I usually return the carp to the water.

Thanks for the info.