usually, if it's the same as my '98 1.8T, underneath black plastic elbow on water pump, with pipe then going up into engine bay area somewhere, becareful if yours is the same as the plastic on mine, it can be a bit brittle and break. if memory serves me right, then 2 or 3 10mm bolts.
thermostats are cheap and to be honest if you are draining coolant etc to get to it i would just put a new one in instead of testing old one, could save time and money later.