Due diligence.
WHAT TO CHECK
Last updated date Active installations Recent reviews, particularly negative ones Speed of the demo site Whether it works on a phone What happens to content if you switch away
HOW TO CHECK MOBILE
Open the demo on your own phone.
Not a desktop browser resized. An actual phone.
HOW TO CHECK SUPPORT
Read the support forum. How quickly are questions answered, and are they answered at all.
An unanswered support forum tells you what to expect.
THE LOCK-IN CHECK
Does it use standard WordPress content, or its own storage?
Ask directly, or search for people who switched away from it.
WHAT TO TEST BEFORE COMMITTING FULLY
Install on a staging site Build one real page See whether you can achieve what you need
BETTER THAN COMMITTING AND DISCOVERING
WHAT TO AVOID
Themes not updated in over a year Themes with unanswered support questions
