Some television retailers, especially the cheap and nasty ones, like to fiddle with the settings on televisions to make some look better than others. Sometimes they're trying to drive you to more expensive models, sometimes they're trying to drive you to the brands offering the highest margins or best "bonuses" for hitting sales targets.
A common trick is to crank up the brightness, contrast and colour on some televisions to make the picture more vivid. Of course there are some aspects of picture quality that you can't easily hide, so stores also look for content such as slow moving HD promos, or animated Pixar movies, that make any television look great. If they want to make a TV look bad, they tend to deliberately misconfigure the aspect ratio - stretching or squashing the picture - and even switch to an analogue channel rather than crisp digital.
The torture test for big televisions, especially LCD panels, is fast- moving sport. No amount of tweaking the settings will hide massive pixelation or horrid motion blur. Head down to your local retailer when there's footy or cricket on the television - preferably something that's being broadcast in SD and HD so you can compare. Ask the sales assistant to flick all the televisions on the wall over to the game.
If they refuse, ask them what they're afriad of and threaten to take your business elsewhere.
Once you're watching the game, pay attention to the little things. Don't worry too much about the close ups of the players' faces, as these generally tend to look pretty sharp. Such scenes might be useful for judging skin tones, but of course it depends on how much the store has played around with the brightness, contrast and colour.
Pay more attention to the wide shots of fast-moving action. Does the grass turn into one green blurry blob as soon as the camera moves? Are the lines on the field jagged or smooth? Does the ball actually look the right shape? How blurred do objects appear and are the players surrounded by a halo of pixels when they move? How clear are the signs around the boundary and the faces in the crowd? It's small details like this that will help you judge the qualities of the panel and the backend video processing. You generally find that plasmas come out ahead of all but the best LCDs (such as the Sony Bravias).
It's also worth taking along a DVD movie that you're familiar with and asking if you can watch a few scenes. If you're comfortable with the fundamentals of picture calibration, you might want to grab the remote and tweak a few settings.
One of my favourites scenes for testing is in The Lord of the Rings: The Fellowship of the Ring when Gandalf first visits Isengard. Poor televisions often struggle to distinguish the fine whiskers in the wizard's beards, leaving a blurry patch under their bottom lips. The fine details carved into the furniture in Saruman's chamber are lost on televisions with poor contrast and severe black crush (in which the darkest shades of grey are all displayed as black). Also look for scenes where the camera pans up and down to see if you get slight stuttering in the picture.
Such simple tests go a long way to separating the wheat from the chaff when you're shopping for a new television.