I have found a very interesting way to tell Cocoa apps from Carbon apps. First you have to set the dock to always show. Then drag the window of the application in question down to the dock area and past it. If the window stops and remains at the top of the dock, you have yourself a Carbon app; if it continues past, then you have yourself a Cocoa app (though the windwo will spring back to the top of the dock when released).
[
robg adds: We've had a couple of previous hints (
1,
2) on distinguishing Carbon from Cocoa. Personally, I still don't usually care, but I thought this method was interesting enough to add to the archives...]