Yes, you must check the LCD screen as it is the most expensive part. A very simple test. Run a program like PowerPoint/ Keynote or something similar, put a black background and run the slide. Your screen will should appear fully black with no white pixel (or in layman's term, white spot). Then run a white background slide and run the check again and this time there should be no black pixel there. If there's any, its up to you whether you want to go ahead with the transaction. Usually LCD screen with dead pixels will degrade further in time.