normally there will be plunger (usually protected by protective cap. near the Buchholz relay and you can see the movement of the mechanism from the sight glass. If u have two stage of tripping then you have to press the plunger slowly down for stage 1 trip and further down for stage 2 trip. Via a aux contact from the buchholz measure the state of the contact as you press the plunger down.
The proper way to test a Buchholz unit is to blow air in using a foot pump (not a compressor!) through the gas vent. Be sure to open any valves to the conservator first though and don't do it with the transformer energised (obviously).
This is the only way to check the integrity of the floats and pivots.
In order to test the oil and winding temperature indicator I recommend to read the instrument manufacturer instruction or to contact him if you have not. For example see:
