At home, with your current TV and DVD player, use a calibration DVD such as Avia or DVE until you are sufficiently familiar with it that you can do basic, good-ballpark setting of whites, blacks, colors, and sharpness relatively quickly. I'm not talking precise perfection here -- just pretty close pretty fast. It should only take a couple minutes to get things dialed in like this once you are familiar with how to use the controls on the set and how to use your calibration DVD.
Pick a smallish collection of DVD's you like and pick a few scenes from them as test scenes.
Go to the web site for the display's manufacturer and try to find an online version of the user manual for the display(s) you are interested in. Using the online manual, familiarize yourself with how the remote control works, and how to do picture adjustments in the menus for that display. Pay particular attention with how to turn off torch mode features such as overly bright, contrasty, red, and sharpness processed "picture" modes. If you skip this step, you may find yourself floundering around a bit in the store as to how to adjust the set -- don't expect the sales person to be much help.
Read up on your displays ahead of time (on this forum for instance) to find out if there are any potential problems you want to check for such as insufficiently dark blacks.
Grab your favorite DVD player, a set of cables, your calibration DVD, your selected set of test scene DVDs and any notes you've made about what you want to try and how to try it. Go to the store WHEN IT IS NOT CROWDED. If at all possible, pick a store which has the displays you are interested in set up in a room where the lights can be dimmed.
At this point it should be fairly easy to convince the store people to let you cable up your DVD player to the display and try stuff. If not, pick a different store. Be sure to note the CURRENT settings they have for the display and reassure them you will reset it just the way they had it before you finish. The hardest part of all this may very well be their hunt for a remote control that works for that display along with batteries that still have juice.
Now use your calibration DVD to set the display to a known level of calibration. Again, be sure to turn off any torch mode stuff BEFORE you adjust the calibration levels.
Now play your test scene DVDs and see what there is to see. If you are lucky enough to have two displays you are interested in side by side, calibrate both of them first and then play scenes on one and then the other (you will likely have to move cables). Be sure you are using the same TYPE of input (Component cables for example) and the same resolution to make a fair comparison.
When done, don't forget to return the displays to whatever godawful settings the store originally had them set to.
Doing your homework and then going into the store equipped like this will at least give you a fighting chance to see something worth seeing.
You may need to go home and think about what you've seen and then go back to the store and try it again just to be sure. Also, keep in mind that sometimes stores unwittingly put faulty equipment on display, so if you see something bizarrely wrong compared to what you expected from your research, it would be best to confirm at another store.
Even if you can't see your chosen displays in a darkened room, testing them this way is still *WAY* better than just trusting to whatever settings and hookups the store uses by default.
--Bob