Contrast ratio is nothing more than a gimmick and response time on the screens we get is the same- 5ms response time overall and 2ms response time for grey-to-grey.
#1. Check for ghosting. Only way to check it is in the darkness. Turn on the lcd screen without being plugged in to the system. Do note that TN panels (google for different type of LCD) always have ghosting issues- so lesser ghosting effect the better.
#2. Warranty- people just see the price and they buy it. You be a smarter person and see the duration of the warranty. Do note that you will need the bill to avail warranty.
#3. connectors. DVI and hdmi are practically the same, only difference is hdmi can give out sound on the same cable but no1 uses speakers on the screens since they are way too flat. Just letting you know because dumbarse dealers pitch their products this way.
#4. Dealers also have a habit of saying that this screen is bad when its cheaper and/or sold out. Be open to go other dealers.
5. lcd screens are supposed to be sealed. If its opened, you have the right to refuse since its tampered with.
6. For checking dead pixels, its the same way as point over one, but instead of seeing ghosting effects, see if you can see any minute discoloured white dots. Do note that certain companies only replace screens with dead pixels with atleast 4, whereas some good companies like dell and benq consider it as DOA and dealers give immidiate replacement- provided it is done within 2-3 days.