[Boron]

if you have a patient enough friend you could try it :
battle 1 : attack with 200 imps
battle 2 : attack with 100 imps + 100 vampires

if the 200 imps routed when they were mowed down to e.g. 100
but the 100 imps + 100 vampires didn't rout until only 50 or 40 were left than this would strongly speak for an averaged morale .

but vampires have only a 1 higher morale : 10 compared to the imps 9 so they aren't good for the test . and only immortal commanders never retreat or ?
so the test would be much better with devils and imps because devils have 30 morale so an averaged morale should be at ~ 20 for devils and imps in equal proportions .