Plasmas also typically have fans to cool the display, so noise level is a dead giveaway.
Hmm, that's true as well, although the fans on my Pioneer are so quiet I have to stand next to the TV in a quiet room to hear them. And I used to have a Sharp Aquos LCD that also had fans - they were comparitively much noisier than the ones on the Pioneer.
Another 'feature' of my plasma is that if you hold your hand up close to the screen, you can feel the heat radiating off it, seeing as how it uses nearly double the juice that my Sony LCD TV's use. Comes in handy during the cold winter days :P..
But then the cinema picture on my Pioneer just puts the Sony's to shame, so to me the extra juice, weight, etc is worth it..