How to find an unagressive female betta at a store?

[nekura]

I had a great sorority going but my alpha and non-agressive girl died. She kept all the others in line and was basically untouchable in scraps somehow; she also never started fights and was reluctant to fight back but when she did the other betta was quick to surrender. Without her my sorority had to be separated because I don't have enough females to balance out the aggression from some of my more aggressive ones. I really need a new girl that's non-agressive but I can't tell in those tiny cups and it seems like the bettas can't see each other so I can see who will flare at others and who won't. The pet store is an hour's drive away from me so bringing back the ones I find to be too aggressive is too time consuming when paired with my new job. Is there some way to tell? Take a mirror with me? Ask the associates to put some clear water so I can see who flares at the nearby cups?

[laoshun]

It is quite hard to tell, because most females that see other females will flare right away but then will become submissive (vise versa). The only way to tell is when you bring the female home.

[nekura]

The girl I had never flared, even on first encounters. She only flared after another bit her, then promptly put them in their place and stopped all aggressive action. Meanwhile, I have a baby flaring at the huge ones in a nearby tank.