Once you assemble the valve you should run a hydro (shell)  test to 150% of the MAWP ( 427 psi for a class 150 valve, 1110 psi for a class 300 valve, etc.) with the gate valve OPEN and correct any leaks found. See ANSI B16.34 for specific ratings for different materials.   Gasket and packing leaks are easier to correct than pinholes in the casting.  Repair/correct any leaks and retest the valve until no leakage hrough the pressure boundary is observed.
Then close the valve and do a seat leak test to the procedures dictated by whichever standard is applicable: API 6D, FCI70.2, MSS SP61 are a few standards.
Relief valves are a different critter, and they have to be tested for opening pressure, lift, and seat tightness. If they are steam valves they have to be set on steam. Gas valves can be set on compressed air, and liquid valves can be set on water.  Refer to ASME section VIII for safety-relief valves or ASME section I for Safety valves (on fired pressure vessels)
All these standards I have referenced are USA standards.  If you are not in the USA, local standards may exist that apply to the situation.

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