not an app, a site. It checks al mons at once and is superfast when it comes to sorting out your mons.
I just catch as many pokemon as i want, and once i’m full i just log in, and within 15 minutes i can easily transfer 600 mons knowing for sure what IV they had. No accidental 100% pidgey transfers. and even better, no manually checking each catch, just login once and see all 1500 mons in one fell swoop.
It does require a login, which turns off most people to use it, but i’ve been using since the start and there has been no confirmed ban ever.