Hi there, I am considering getting a used 5d2 on craigslist, and aren't really sure what i'm supposed to check. sensor, what else? and how do i even see the sensor? on my XSi i did the mirror lockup thing, but when i look back there all i see is this gray matte plastic thing that doesn't look like the pictures of sensors that i've seen before... is the sensor directly there when i look in the hole in front of the body? thanks in advance!