Pitchbook and prequin are some of the few sources for returns data which shows how financially successful a firm is. For anonymous ratings check out thefunded.
Directly asking entrepreneurs whether they liked their VCs is probably the best way. If certain names keep coming up as great then those are likely the folks worth speaking with. Of course certain firms are thought of as tier 1 and legends in the industry, but that doesn't mean they are right as partners for you.