Tire Hum or Wheel Bearings?

My '88, 420 SEL, had developed a low frequency hum that sounds like a tire but after researching the subject, here, it looks as though it may be a wheel bearing. The sound seems to come from the front passenger side. I'm going to have the tires rotated and if the hum continues then I'll assume it's bearings. It seems that I've read that some tires have a wear indicator designed into the tread but I don't know if that's a visual or audible indicator. I'll appreciate your comments and advice. BTW, the hum is more prominant at low speeds and become nearly inaudible at highway speed (70-75 mph).
Jerry