grounding the alternator

When you upgrade the big 3, how do you know there is a good ground return path to the alternator housing itself?

I mean, the bracket is usually painted in some cases and the housing often looks slightly or moderately corroded. Am I making any sense?

can u run a wire directly to the housing?

I see you are doing some thinking about how a connection is to something (on paint and/or corroded surfaces) and that's good. Not too many people think about those things. Ideally you want a proper ground connection to the alternator housing for the best transfer of power to the battery (and the same on the power side) providing they are both done correctly. Some alternator housings have threaded holes in them for easily adding a connection while some don't. If the surface area around the threaded hole is corroded, sand it down and then once the connection is made maybe apply a material over it to slow down the corrosion. 

Having the ground going to the alternator case may affect other things on the motor and their reliance on grounds of say like the starter. The starter is bolted to the block, the heads bolted to the block, brackets off of the cylinder heads or block bolted to the alternator and then the starter ground continues back to the battery and you can see how all of these connections may affect the starter's performance. You also have sensors on the motor for things too. I really doubt having the ground connected directly to the case of the alternator would affect anything else on the motor but keep that in mind later if the starter starts acting funny, sensors giving wrong readings, etc. It could be to things bolted onto others which are bolted to other things and then to even more things. All it takes is one weak or non existant link to give you problems.

Jim