So, imagine it like the difference betwee drink driving and dangerous driving.
For drink driving, there is a test. You either pass it or fail it. There isn't generally any room for opinion about it.
For dangerous driving, there isn't a test. It is a matter of the guard giving evidence of what you were doing, maybe cctv or whatever, and a Judge deciding on the evidence whether or not that amounts to dangerous driving.
There isn't a test (like a breathalyser, blood test etc) what we can do to determine is someone is incapable of giving consent, so it is a matter of the Judge/Jury listening to the circumstances and deciding whether the person was capable of giving consent or not.
A major issue with it is blackout. If someone is blackout drunk/drugged they may be able to say that they consent, but in reality are probably not capable of consenting.