There hasn't been much research into people who can't trip. There are a few of these people out there, searching google has several threads in all the message boards about that friend many of us seem to have who is immune. I agree you cant decide you are hardhead without a bunch of tests. I had a friend who had no reaction to huge acid and mushroom doses, he gave up trying. A case from the US government secret trials seems to point to at least one person immune to LSD
http://www.newyorker.com/online/blo...rmy-experiments-with-lsd-in-the-cold-war.html
"There were also some unexplained reactions, affirming LSD’s capacity to trigger infinite behavioral variation. One subject seemed to be immune to the drug—an “object lesson in humility,” Jacobson observed. The team began to fear that someone had given the man a warning, or that he had been trained in countermeasures to LSD, suggesting that the whole operation was somehow “compromised to the opposition.” Several weeks later, the team found a pretext to covertly slip him LSD again, this time at more than twice the first dose. The reaction was similar: scarcely any effect. "