The older it is, I mean the further back you can trace it, the less likely it is to be fake. There was no money in faking Bukowski until the internet blew up.
For what it's worth, the writing and signature look beleiveable to me.
And you can apply a bit of logic to these things as well. Meaning if you were going to fake a sketch, would that be the one you'd fake? Or would it be a man with a jug, Bukowski's "signature" sketch?