If think the breakers aren't tripping correctly on ground fault you
could kludge a temporary 110V outlet to each hot-pole with the neutral
to the load side neutral for that breaker and use a commonly available
GFCI tester in the 110V kludged outlet.


The kludged outlet also requires a ground for a GFCI tester to work.


Note there are a number of requirements for a hot tub (US-NEC). Without
looking it up, what comes to mind is:

no outlets within 5'
special requirements for lights within 5' and over that area
exposed metal within 5' that could be energized to be bonded
disconnect within-sight-of but over 5' away (for maintenance
- subpanel would work)
GFCI outlet between 5 and 10' away
if outside no wires overhead (may extend to side)

Bud--