Where is a good starting point for troubleshooting an engine that idles too fast?

start by cleaning out your throttle body by using some sort of carbon blaster!!! You could have a sticky butterfly, or faulty IAC. Or Airflowmeter!!

A good starting point is Key west Florida. If you have only noticed this change in the last 3 or 4 months, it may just be the engine running rich to warm up faster. The numbers you are seeing aren't too far out of wack. I know when you bought it, 500 RPM was probably typical. This would not be a huge concern to me.

The computer is controlling the idle. The computer thinks there is a reason to idle faster. It could be a vacuum leak (additional air getting in the engine), it could be low engine temperature (bad thermostat, bad engine temp sensor, cold outside), a sensor reading out of spec (Probably O2, maybe Mass Air Flow), other load on the engine (failing water pump, Air Conditioning compressor, frozen idler pulley).

Check your codes again. What's your gas mileage like? If you mileage is really good, it is a vacuum leak. If you mileage is very low, it is an O2 sensor.

Check to MAS and make sure it is working. I usually buy one then return it if that isn't the problem.

taking to garage and getting it tuned