The bark is somewhat similar to sweetgun, but the fissures between the corky bark are usually not as deep.
Another great way to tell the difference between sweetgum and elm is the size of the twigs on the outer parts of the limbs. Sweetgum has a larger twig out to the bud, while elms have a thinner (whip-like) twig. I know this sounds silly, but there is a very big difference between twig size for the two species and it helped me back in Dendrology class while I was persuing my forestry degree. If you can ever find a sweetgum and an elm beside one another in the woods ... the twig size out on the branches of the crown will be very obvious.
Also, look for the leftover gum balls on the limbs to ID sweetgum. If they have all fallen, it won't help you much.
The twig size in the winter is key though.