I have a feeling that my camera isn't metering as well as it used to, and I think it is now over exposing by about a stop. Whilst this can be counteracted by using exposure compensation, it's not a critical issue; but how can I check the calibration of the inbuilt metering?
I'd suggest to take an image of evenly lit area without details (wrong focus is a great way to reduce details) and check the histogram. You should see a peak in the center or just a bit left from the center if your meter is correct (left image on the example below, default exposure) and more to the right if your camera overexposes as you suspect (right image on the example below, overexposed 1 stop).
If you have control the light, the easiest way to do it would probably be to compare the camera meter against a hand-held meter and see if they agree or not. Your other option would be to take it in for servicing and have them check it with their computerized gear.
You could try following the "Sunny 16" rule to determine the correct metering during bright midday sunlight. The rule generally follows as such:
1/ISO shutter @ f/16 aperture = bright daylight, distinct shadows, midtoned subject
If your ISO setting is 100, you would use a shutter speed of 1/100. Take a shot, then compare that shot to one metered by your camera under the same conditions. Under aperture priority mode, your camera should choose a shutter speed of 1/100, maybe 1/125. Under shutter priority mode, your camera should choose an aperture of f/16, or maybe +/- 1/3 of a stop.