This is a good thing to remember, as long as your local Best Buy or electronics store doesn’t mind you plugging stuff into their sets. Just get a USB stick (or better, two or three) and fill it up with pictures that will show off the strengths and weaknesses of the displays you’re considering. It’s a bit of a rough-and-tumble way of checking, since the color settings and such might not be optimized, but put a few test patterns or familiar photos up on there and you’ll be able to spot issues easily. Bringing along a movie you know well is another option, but not so good for comparisons. Some more tips here.