How to tell when a cab is available

I just read something that reminded me of when I drove a cab at night in Tampa and would get flagged many times when my cab was all ready occupied.

I would turn on my inside light so they could see that yes I already had a passenger, and at that point I would get cursed out more often then not, it would all start all over again as we passed the next bar in Soho.

I would often tell my fare if I ever get rich Ill take out a full page ad in the Trib and explain when the light is out you need to find another cab, why is it people who go out each and every weekend can't figure that out?