I think the best solution is if you can find the make and model of that motherboard. The vast majority of computer component manufacturers have good websites that will include things like the manuals for past products. In my (limited) experience, many of the motherboard manufacturer's manuals or online support will contain the information you're looking for.

I don't think there's any standard, at least not any that's well known. The pinouts shouldn't be all that hard to find.

FewMiles..