To tell if a user has accepted an invite please look at the 'Can Login' column on the user list (found by navigating to 'Settings' > 'Agency Contacts' > 'Users') as shown here:

No - Means the user has not been invited. If you open their profile and look at the note at the bottom you will see this:

Invited on 00/00/0000 - Means the user was invited on that day and either has not accepted OR has had their profile put on hold by yourself. Please see how to tell the difference below from the note on their profile:

If a user has not accepted:

If a user has been put on hold (please note that this can be done whether they have accepted or not. To check if they have accepted once you have un-ticked this please update and refresh the page): 

Yes - Means the user has accepted the invitation and if you open their profile you will be able to see when the invitation was sent: