Hi Dinky
It all depends on why you are testing. Hopefully the following should have been covered on your course. To test the strength in any muscle you Load it, so have your client perform the action of the muscle ie, you get your client to retract their rhomboids whilst you apply 'resistance' to the move ie. making it harder for you client to make the move. In my experience you wouldn't normally need to test the 'strength' of rhomboids as these are protected in probably 90% and therefore weak and long (kyphotic posture) Traps you could load in sitting, prone or supine asking your client to shrug whilst you apply 'resistance/pressure'.
NMT is just a static pressure which again I would have hoped you learnt on your course. Pressure is easiest to apply with tools or elbows. Resiting on the spot for up to 1 or 2 minutes but ultimately checking in with your client to see when their discomfort subsides or you feel the muscle 'release'. Again not sure you'd want to use NMT of rhomboids unless they really are retracted and tight, but your over the ribs etc. NMT great on Traps, seated supine or prone. Hope that helps.
M
Again