How to judge the fame of your potential advisor?

[Joe_Fraizer]

I hear that how well known your advisors/recommenders plays a big factor when applying for PhD and the life after the PhD with said prof.
But what counts as a well-known or famous professor? I am asking because I want to do a PhD with a professor at University of Hawaii who I know personally and I like him a lot, but I have a good enough resume to get into a top 40-30 University.
Maybe his reputation would counter the not-so-stellar reputation of UHM?

[bfollinprm]

I want to do a PhD with a professor at University of Hawaii who I know personally and I like him a lot

The most important thing when getting a postdoc is publication history. If you and this prof work well together, then it shouldn't matter the reputation of him or the school; you should go work for him. Making decisions like these will make your PhD infinitely more enjoyable, and if you have a good relationship with your advisor it will be much easier to get papers out anyway.

[King Vitamin]

The best bet is to figure out where their former students went. This usually comes to water cooler gossip type conversations with other students (though I suppose it shouldn't be impolite to ask a potential advisor where their students went).

EDIT: I'll add that I've heard stories of very famous professors getting their former students subpar postdocs.