The smooth softshell looks very much like the spiny softshell. The best way to tell them apart is by looking closely at the front edge of their carapace. In the spiny softshell, tiny "spines" exist, while they do not in the smooth softshell.
Pictured above: This is a Smooth Softshell. Notice how there are no spines along the front end of its carapace? Now compare this with the Spiny Softshell below.
See the spines?