Same as for the reporting of any other crime (burglary, theft, assault, murder)... you proceed from a position that the complainant is making an honest report, then you set about investigating the veracity of that report.
How would you like it if you called the Police and told them that someone was in the process or burgling your house, and their response was "do you have any evidence to prove it?" or "you left your house unlocked so you deserved it?" or "you must have pissed the burglar off somehow so its your own fault?" This is effectively what happens when you dismiss a rape complainant's allegations.
I don't get this mindset that when a woman is raped, that some men (and I have to say, they are mostly older, right-leaning types) fail to understand that rape, attempted rape or sexual assault are a very traumatic experiences for the victims - they have been personally violated. There is a well documented and well researched history of people and authority figures such as Police and College staff not believing the victim's outcries. When that happens, the trauma is magnified - not only have they been physically violated, but they have also been emotionally traumatised through being disbelieved.
IMO, it is the potential additional trauma resulting from being disbelieved, as well as the shame associated with having to explain to a stranger, often in graphic detail, what happened to them, that causes women to so often not report that they have been sexually violated.
