If you suddenly realize you are running naked on a freeway with a balloon in your hand, there is a good chance SOMETHING has happened to you. Whether it's hypnotism or not is hard to say at this point.
It is not easy to tell whether you have been hypnotized due to vague memory. However, there are those who remember incidences of hypnosis. Sometimes, hypnosis feels just like your normal state of consciousness. You can visit the site http://aaph.org/node/271 to read more details.