Best way to check is to pull a wheel. Like Scott said, wear indicators are left front ant right rear and the sensor depends on a complete circuit through the sensor so if it got unplugged you'll get the warning message.

I normally have one set of rotors last through two or three sets of pads, and changing just the pads is an easy job, no exotic tools but be safe and use jack stands. If the rotor is not grooved, I don't have it resurfaced--just put in new pads and use the brakes easy till they bed into the surface of the rotors.