For better or for worse, proving sexual assault beyond reasonable doubt is extremely difficult to do. In most cases, there are no witnesses and the alleged attack occurred in a private room. In other words, it will usually be one person's word against another's. Even a rape kit test will usually prove only that sex occurred, not necessarily that a sexual assault occurred.
A rape kit test could also indicate mind-altering substances in the body, but again, it won't necessarily indicate whether the person took the drug knowingly or not.
Proving consent or lack thereof is the part that's extremely difficult to prove beyond reasonable doubt.
