My Mega Drive II: (Still applies to the Genesis II as they still use similar motherboard revisions)
My MD2 is sadly one of the bad ones, and thats a picture of the bottom of it. (I don't like that MD2, Its just in storage, I use Model 1's)
VA0, VA1, VA2 and VA2.3 models all look like this on the bottom (Mine is a VA1, I also have a VA0 MD2 which looks identical) all of them have various audio issues and you should avoid them if possible.
Another Sega MD/Genesis Model II, picture taken from Sega-16 pic is not mine - ignore the circles. This is a VA3/VA4 model which have the better audio (Although still not quite as good as a non-VA7 MD1)
Notice how the RF shield that you can see through the vents is smaller and doesn't fill the whole case like in my Mega Drive 2? The VA3/VA4's have smaller motherboards than the others and that's how you can tell the difference without opening them up.
Happy to help
Edited by R.Bear, Fri Sep 12, 2014 6:07 AM.