I clean my own bottom (ok, we all do!!). J/K. I dive mine except when the water is cold. I have this strong aversion to diving in the cold and I hate wetsuits. So once twice in the winter I have a local diver take care of it.
I can tell right away if he (or I) did a good job. With clean bottom at 2000rpm, engine is running 11psi boost and speed is right at 20kts. If dirty, same rpm will be like 13psi and speed lower. After his work, speed and boost are right back there to the "clean" numbers.
I have since hauled the boat and added some pitch to the prop. Now 2000rpm is about 22kts and boost is STILL 11psi. I am sooooo happy!!
If you know your clean bottom rpm/speed correlation, you will be able to judge how dirty the bottom is.
Be careful with divers doing monthly cleanings. Many stories of divers getting a little slack and not doing much of a job, especially on boats that sit. But they still bill for it. Hauled one boat that was on a monthly program and there were huge barnacles and sea squirts on running gear. Diver got a call while it was in the slings. He showed up and was embarrassed as all hell. A cash refund was made on the spot!!
I take my ride to a slip where the diver works and generally am on the boat as he works. No opportunity for slackness there. And this diver has a good rep too.