Generally you do not feel any different, but are more suggestible. The hypnotist can include "convincers" that will prove to the subjects that the state has been altered. These may include suggestibility tests or imagery altering. Normally, the subject simply gives temporary control of their environment to the hypnotist. The level of awareness is the only real measure, since the hypnotist guides his subject to concentrate on their thoughts and body. External stimuli can trigger a desired response, as in a stage show to achieve hilarious results.