For Starling Bank
To check you have Flux activated, simply open up the Marketplace in your Starling app. If Flux is activated, you'll see it on the main page - and it'll show the word 'connected'. If you can't see Flux on this screen, then you'll need to add it to your account.
To check you have Flux activated in your Monzo account, you'll need to navigate to Settings-> Privacy and Security -> Manage apps (in the Monzo app, not your phone settings!). You should see Flux listed under 'Third Party' apps. If you can't see Flux here, you'll need to add it to your account.