Pretty straightforward: you send an event for which variation your user saw and another when they login, attaching the login method to the attributes.

You can use Mixpanel or Heap, which have mechanisms for mapping the non-logged-in user to your verified user on login, though you might need a bit of custom code to do it.

Ah. We use June.so ; I'm assuming they might have a similar mechanism. We'll try and log login attempts so we can track this. Thank you!

No worries! Our experience is that the hardest part (and it's not trivial) is associating the "anonymous user ID" to the newly logged-in user. Segment has an identify() function (I think Mixpanel has something similar), where you link the aliases, which then makes the reporting work.

I've not tried June, so I can't say for sure, but it's a pretty common feature for product analytics. I'll be surprised if it's not possible.