It really depends on the individuals. As mentioned, Euphyllia genus corals are very similar and are all "strong" corals and will tend to out-sting others. Sometimes two Euphyllia live happily next to each other, never fighting, other times it's all out war. In my experience, the "strength scale" for lack of a better term amongst Euphyllia would be Hammer>Frogspawn>Torch. But again, each individual coral is different in this regard. I have two torches, one really bright green, the other a little more dull. The really bright one kills everything around it, even went after the other torch once... However it doesn't fight with the frogspawn right next to it. Very wierd.
If they're attacking each other you'll know cause their tentacles will stick to the other while attacking.