I know of no test either-
until you just "know" by feeling the traps spring strength best way would be to open and close new trapsthat way, you get a feel for what is acceptable.
I would guess thats most trappers do it- when a trap "feels' weak, it usually is and springs get replaced.
if in doubt, i'd buy replacement springs- tom here i know sells such for most traps.
one thing to keep in mind- esp double longsprings, can be "weaker than their counterparts in coilsprings- because you need to break both springs down past the jaws, to loosen the trap- coils you just need to "start" them before tey open up.....trappnman