How do I verify that people actually signed up or completed a bonus action?

Some bonus actions are automatically verified.

Here is a list of those actions:

https://vyper.zendesk.com/hc/en-us/articles/360033504192-Which-bonus-actions-can-be-reviewed-automatically-through-the-API-

For other bonus actions, you can enable the "manual review" when you're editing the bonus action.

This will allow you to ask a question that the contestant will answer and you can review whether or not it's acceptable: