Before releasing a theme I use the checklist below to ensure the theme is fully tested and prepared. There's a lot of things to remember and this makes it easier to ensure that I have covered everything I need to.
How To Use
To use the checklist I copy it into the top of my themes functions.php file as a block comment. I then work down the list, one item at a time, deleting the item as it's complete. This gives a quick and easy way to tell how much is left to test.
test theme with and without Jetpack
test theme with and without infinite scroll
delete unused scripts
delete unused customizer controls
screenshot (880 x 660)
check custom header size is set correctly (and other custom image sizes are set correctly)