I purchased and downloaded Limewire Pro. I see no difference between the two. How do I know if i'm running pro and what difference shall I expect?

No ads, email support and the fact you show up as a supporter of LW development. The free works just as well.
You can see the version in the "About LimeWire" box. (btw, LW just caught a minor OSX bug in the About" window that is fixed for the next release. Just close the "About" before clicking on the dock to avoid the bug for now)