Buds get pointy like a paper rolled into a cone...
It's not a definitive characteristic, but American elm will commonly produce reddish trunks buds when you chop them for bonsai. Neither cedar elm nor winged elm do this, though the growing tips of their shoots will often be reddish (while American elm's are not, go figure). Just something I use to help tell them apart. The classic shape of the American elm leaf is generally a dead giveaway.
Dutch Elm Disease is not a reason to avoid this species as bonsai. Far from it. It can make terrific bonsai. Should be used more. There are several DED resistant strains of American Elm out there, like the "princeton" cultivar developed by The Dept. of Ag and other sources. Even older trees can fight off the disease with some help. All the trees on the National Mall in D.C. are mature American Elm. They were treated every few years for DED to keep them going. Don't know if that program is still underway though.
