After clicking someone's invite link and downloading the app, you will be shown the profile picture and screen name associated with the person that invited you. The app will tell you that you were invited by that person. If you do not see that message, then the tracking was broken and you should try clicking the invite link again.
To check if you have a referrer, post account registration, here's what you have to do:
Go to Menu (☰)
Press "My Network"
Press "referred by"
If you have a referrer, it will show on the Referred by Section.
If you do not have a referrer yet, you can manually add one post registration, here's how:
In that "referred by" view, enter your referrer's code in the appropriate box. Referral links are in ALL CAPS.
It is important to note that if you get any children/downline before you complete this action, you will no longer be able to complete the self-movement and you will be locked into your current position. So please follow these instructions as soon as possible to avoid getting auto-filled people in your network, and do not start inviting until you have added a referrer.