Makers paying creators for your app: how do you tell who brings paying users?

Lots of app makers here run creator or UGC programs alongside their launches, so figured this is a good crowd to ask.

Views are easy to count. Installs mostly too. The tricky bit seems to be knowing which creator brought the people who actually subscribe, and who's still around a month later.

I've seen promo codes, per-creator links, and a lot of "the chart went up on Tuesday, must have been that video". All of them seem to work until you have more than a handful of creators.

For those of you who've been through a launch with creators involved, how did you figure out which ones were worth keeping?