Can you tell if some one is a rapist or murderer or Kidnapper?

Maybe if the guy says take down your pants he might be a rapist or a doctor with a grudge from a prostate exam.....and if a guy points a gun at you he's probably a murderer or a guy with a bad case of morning wood oh and if a guy asks you if you want a piece of candy to get into his car then he might be telling the truth or he's a kidnapper! but you know I'm not sure or anything...