Screen stays black after falling on the ground and the water

I have an iPhone 3G a friend to repair.

He said me that it fells to the ground several times that broke the glass. And once it fells in the water too. He tried to turn on and no sign. I cleaned and replace the broken glass. Now, iTunes will recognize it and the iPhone is the beep when plugging the cable. The only problem is that the screen stays black, the vibrator does not work and I do not know if the iPhone starts correctly.

Is there a way to test the screen or to know what is not working properly? Is it the backlight?

Thank you in advence for your reply.