- Check the demo’s performance on GTmetrix !important… From my experience, if you get an F score it is likely that you won’t be able to optimize the site much
- Sometimes author will add admin panel screenshots, make sure to ask for the actual access to be able to check every aspect of it.
- Check the Online Documentation (If Exists)
- Supported Plugins: before buying a theme you should make sure it supports the plugins you will use
- Make sure theme updates are done on a regular bases (at least once every 2 months for older themes)
- Check the responsiveness of the theme demos yourself
- Check browser compatibility yourself
- Read comments (themeforest or theme support forum) !important…
- Analyze the Quality of Support by making sure the author is patient and answer questions in a timely manner,
- Make sure the theme is SEO friendly
If you still not sure you can ask the author some presale questions until you satisfied