I can tell which is which every time! No matter what the size I can easily tell the difference between Plasma, DLP and LCD. Plasma always looks crisper, cleaner and brighter with more vivid colors and much, much , much higher contrast than the other two.
DLP has great contrast but the colors are alway more muted. The lines are no where near as sharp.
LCD always has an after-shadow. Sometimes it's hard to see at first but the first time the picture goes from a bright image to a dark image in a frame, it is unmistakable. Luckily for me my eyes are going and my biological persistance of vision is almost as long as the LCD's after image. But I can still see it. Also the edges of the screen always appear darker than the rest of the picture. The contrast is good, but I have yet to see an LCD that can produce true black. The 10% gray (as oposed to the 7.5% gray that is standard TV black) is something that always makes me cringe.
Unfortunately, the plasmas have not made it to my price range ($0). When one of my TVs finally bites the dust I will get one of these babies, but not yet. It is still too soon.
See ya
Tony