Like Brian B said, you can tell by which Sat source you're using and by the way the feature behaves. Also as Brian B said, WV sat shows you the moisture in the middle to upper atmosphere, so it shows ULL's very clearly. In general, ULL's have very broad circulations. They wrap around for hundreds of miles while surface lows are generally much, much smaller and nearly impossible to see on WV - of course that's unless you have a mature hurricane w/an eye) On the Vis and IR channels you can see from the sfc to the upper levels. Sfc features are easiest to distinguish on Vis, but as your eyes get better at seeing the details you'll also be able to see them on IR. You'll also learn to see ULL on those channels as well.

Also, the ULL that was ahead of Dean and could have allowed him move north did move through the GoM and into Mexico. (I think that's the ULL you remembered being mentioned) A new one formed behind Dean and fed off his outflow. During that time it began interacting with an already present tropical wave and created the mess we see now.