body language can be hard to read in fish, no facial expressions
gill flaring, mouth gaping, holding fins out can be obvious
the swimming position in the tank can be more subtle
fish swimming towards each other and how close they get before turning can be little signs of impending agression
usually an outright attack is presaged by posturing and vying for the best swimming space, or hidey hole
observation and experience will help in your judgements of seeing trouble before it starts
i have 2 aros together, it takes them about a month to confront each other then they have to be divided for about 2 weeks the its back to swimming together
need to acquire a 3rd fish to breakup the one on one confrontation
resident fish will all be taken out and tank rearranged, then new fish introduced then existing fish
this will create a new territory for all fish to settle into
if that doesn't work it is into the big pond and bring on the Pacu's, more aros and some knives