The drive belt on the SR/S is almost maintenance free. Just keep it clean, and check the tension and alignment from time to time. In theory the belt shouldn't stretch much at all, so if the rear wheel doesn't move relative to the swingarm, the belt shouldn't need the tension or alignment adjusting often. To check the tension there's a tool made by Gates called the Krikit gauge. There's two versions of the Krikit, one for tensions between 15kg and 75kg, and one for tensions between 50kg and 150kg. The Zero SR/S belt tension should be between 51kg and 102kg, so the Krikit II (part number 91132) is the gauge for the job.
The gauge is easy enough to use. Slide your finger into the cream-coloured elastic holder, and press the green arm flat with the body of the gauge. Then hold the gauge under the drive belt, mid-way between drive sprocket and wheel, and press up against the belt until the gauge clicks.
Then read off of the scale where the green arm is poking out. You should rotate the wheel and take a couple of readings, although a belt doesn't get 'tight' spots as a chain would, so the readings should be pretty similar to each other.
In this case the tension is about 55kg, within tolerance but quite close to the bottom end of the allowable range. Next weekend I'll take a look at adjusting the tension to bring it up to around 75kg.