It's not hard:
If a woman comes forward with an accusation, you investigate impartially. That is, you always investigate. If her story check out or you find corroborating evidence you proceed. If not, close the case.
In Bill Clinton's case, the dude was put under a microscope for a decade by people highly motivated to find anything they could use to prosecute him and were willing to act in bad faith to do it. If that kind of scrutiny failed to come up with anything to pin on him, then I feel pretty confident assuming that Clinton never raped anyone.
The point about believing women when they come forward with rape allegation is that they should never be dismissed out of hand and should always be properly investigated. And all the allegations against Bill Clinton have been investigated ad nauseam, by people with a vested interest in finding fault in him.