Normally you would use soap water (water with a pinch of soap) or leakfinder in a spraycan.
Apply at the inlet hole first, if there will be a bubble here it is the inlet valve.
Next step is to apply to the muzzle with side lever closed to check if the main valve seals.
Locate regulator if equipped with this and apply the solvent.
After this i would check all connections between tube and action and thereafter the gauges.
Make sure to wipe it of or blow it dry with compressed air afterwards to avoid corrosion.