I created a custom app, using uber sdk (https://github.com/uber/rides-ios-sdk). I also have "connect" button along with singup option and I'm using affiliate program for US-based clients using "client_id" parameter during sign-in/up (https://developer.uber.com/docs/riders/affiliate-program/introduction).
How can I test it, and how can I check if new users will be connected to my account?
Let's assume that I have app in production mode (running in real environment and approved by uber and apple).
That's a good question. Because signups / first trips are not in the self-serve analytics provided on the developer dashboard at developer.uber.com unfortunately I do not think there is really a way to test that a user signup / first trip has been attributed to your app.
As long as you are following these rules, the signups / first trips will be attributed to your app:
"Always link to https://m.uber.com/ul/?client_id=. Always include your client_id in your deep link."
And payouts begin once an app reaches $250 in referral fees (so 50 signups/first trips attributed to an app). This FAQ might answer more q's about your affiliate questions: https://developer.uber.com/docs/riders/affiliate-program/faq