The typical recommendation is to recalibrate a torque screwdriver about every year or 5000 uses. If you drop it from a distance or onto a hard surface, you should also recalibrate it.
Your options are to take it to a calibration technician, or to DIY it. It is possible to buy a bench-mounted digital torque tester, but they cost anywhere from several hundred to a thousand dollars.
The manual DIY process is pretty intense and demands methodical precision — the amateur may find it intimidating. As well, it depends on whether you have a torque screwdriver with a visible or accessible calibration screw. Tools required: vise, tape measure, weight, string, screwdriver/hex key.
Resource for recalibrating torque screwdrivers available here.