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Probably could get a small inspection camera in there. My mechanic uses a small diameter one that links to a phone. Focus can be tricky. At least look through every hole and cover plate.
You can remove the lower inspection plate and look at it, but it usually gets cracked around the 6 bolts in the center and the cracks work their way outward. You should be able to spot the outer cracks as little black lines.
Here is a video from when I suspected a flex plate issue on my 1997 D1. You can't see much of the flex plate. I used a boroscope to examine the condition of the flex plate.