We make it easy to check whether or not a guest has viewed your invitation. Once you've sent your invitations, you'll be able to access the "Manage Invitation" dashboard where you can check your guest list and RSVPs. On your guest list, you'll have three columns labeled Sent, Viewed, and RSVP.
Take a look at the Viewed column. There will be a date in this column that shows when each guest opened and viewed the invitation.
As long as you see a date here, that means the guest has seen your invite. If you don't see a date in this column, this means that the guest hasn't viewed the invitation yet. You can resend them the invitation by clicking the gear symbol next to their name and choosing "Resend invitation".