There is no Preview or Print Button for Document Templates at this point in time. If you want to see how a document looks "live", you will have to apply the Template to a Booking following these steps in order to print it:
Open the relevant Booking
Click on the Documents Tab
Click New Document
Select the Document Template
On the New Document pop up click Save
On the Edit Document page click Preview
On the Document Preview page click Download
On the PDF Settings Pop up click Download
Once your PDF document has downloaded, Print
Additional Information
If you'd like to Print off the Template 'as is' with the merge field code visible, you can print via your browser, (CTL+P for Windows or Command+Shift+4 for Mac) or use a screenshot device to capture what it looks like.