I am not sure about the percentage affected or what the preconditions have to be, or what other measures to detect them are, but personally I never tried psychedelics recreationaly due to their extremely long duration of effect.

I know there is a good argument about setting, mindset and support being the relevant factors, but the possibility of multiple hour horror trips always made me think again. And again. Which in itself is a huge problem for people who get complications because of that fear.

There are some drugs who work similar to an "allergy test". Salvia divinorum for example has an extremely short duration of effect. A few minutes (<10) in comparison to the same amount of hours. With proper supervision the potential to detect people at danger without lasting complications looks extremely promising to me. Sadly I dont think anyone will care for such safety measures as long as the whole benefit of psychedelic drugs is illegal at the moment.