to know when i'm ready for an oil change, i use the odometer setting to record my milage. at every 3000 miles, i give my supra an oil change. i use mobil1 synthetic motor oil along with a toyota oil filter. while changing the oil, i usually give the fluid levels a quick check...windshield washer, coolant, brake, power steering.
when i'm filling up at the gas station, i check on the front and rear tires to make sure they're wearing evenly. just simple stuff you can do in a few seconds.