I can answer that.
I've worked in the with the DD (developmentally disabled population). Some were the textbook image of what we think of as as mentally impaired (helmets, drooling, repetitive movement, often times physically deformed).
For the most part those with the highest degree of retardation (moderate or profound) showed. They were mentally children or babies, hence being unable to consent.
Some were high functioning. They could get up to borderline (around 70 IQ), look normal and can fit in a normal crowd of people, shop, commute, hold jobs, etc. and you couldn't tell. Of course, if you talk to them for a while you'd see they're a bit "off" but how many typically developing people with no retardation act "off"?
These higher functioning ones were usually deemed able to consent to sex, have driver's licenses, jobs, etc.
Hopefully no one would ever need to worry about making that mistake ever because for those who cannot consent; it's pretty obvious that they cant consent either because they look it, or because they're in some sort of group home or something so you'd never need to worry about that.
If you think about it, this sucks because the intention is to give these people "normal" lives as best as we can and not restrict them from enjoyments that "regular" people would otherwise be allowed. However the topic of sex is a tricky one so they'll never be able to get laid their entire lives... not legally.