I look at H-index and citations per journal article. I actually tried to find out whether there was any correlation between the type of research/ideas a professor has, how well he deals with students, funding etc with his h-index. While I dont have any quantitative results (), it seemed the correlation is good.
However one of the problems with this approach is that this is good mainly for associate professors or above, because assistant professors really dont have the necessary time to get good quality journals published and referred. In that case, a good metric of future performance for assistant professors will probably be no of citations that their journal articles get per year per article, total funding, quality of journals.