Trying to control it starts with you realizing the difference between reality and being in a dream state. Eventually over time, if you keep a record of your dreams, you'll get a pretty good feeling of when your in a dream or not. There is no one particular sign that a person can tell you, its just a feeling you YOURSELF know. So I suggest every morning once you wake up record everything, while your memory is fresh. As many details as possible of your dream, even the tiniest details eventually help, as you'll find a pattern of sorts. I wish you the best of luck. Its seriously an exhilarating experience once you get a hang of it. =)