How to tell if Digitzer is bad?

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http://androidforums.com – Hi, I sat on my Samsung Galaxy S Captivate last night and most of the screen went black with just part showing, but no cracks on the screen are visible. I was able to put the phone into flight mode based on what I could see on the screen. This morning however there is no display at all on the screen. I see that I can replace the screen and the digitizer. Does it sound like I need to replace both or just the screen, or just the digitizer? Frank (General)