If you have a step in your tree where you want to verify the end-user performed an action, you can use the "Require Confirmation" feature when editing a node.

When the end-user is in the tree, they will see a message in red ink like this:

They will not be able to continue until the checkbox is ticked.

Here's how to set this up:

1. Edit the node that you want to contain this message.
2. Click the More tab.
3. Check the Require a Confirmation Click with the Following Text button.  
4. Enter a message to display to the end-user. It will look like this: