If you want to quickly see which Images in your database have no Indexes associated with them, you can run a Missing Index Search:
You'll then see a list of all of your Images with no Indexes applied, as in the example below:
If you want to add an Index to any Image here, you can then add that Index as normal.
Note as well that you have a Refresh button above and to the right of your missing index search results:
If you are going through the process of adding indexes to all of the files here, this Refresh button will help you keep track of which images still have no indexes applied by removing those that you have taken care of.