You can preview your widget in various ways:
Click Widget in the upper right-hand side of the account when logged in.
Click Publish > Your Public Site, and click View Public Site.
When going through these methods, you will be going through the calendar in admin mode as you are logged in to JRNI at the same time, therefore you'll be able to choose from your existing list of customers who have booked with you previously. Customers will not see this.