How to know if a new display assembly is needed

Besides the obvious, ie a physically broken screen, how do I know if I need a new display assembly?

Currently if I turn on my iphone4s the screen dimly lights up, but there are no icons no shapes nor colors. Does this mean my lcd needs to be replaced or is it the logic board?

FYI, the phone was dropped in water. I tore it down, cleaned all parts with 90% alcohol (there were some areas of corrosion) and it's clean as a whistle with a new battery.

The phone makes the charging sound when connected to a charger or usb. When connected to a computer it asks for a password but I cannot put it in because I cannot see anything on the screen. It also seems as though it doesn't register and inputs via the digitizer.

Please help, I do not want to but a new display assembly unless I am certain that is the solution...since it is $130. Nor do I want my 2 month old 4s to be a paper weight.

Any help/advice would be greatly appreciated. Thanks in advance to all the iFixit members.