How could I watch what pages are indexed in Google?

After logging in, go to the Index section and select the Coverage tab. It will be possible for you to create a list of "All known pages" there. You can see a breakdown of the number of your pages that are "Valid with warnings," "Errors," and "Valid with errors" here.

The Coverage tab can be found in the Index section after logging in. There, you'll have the option of creating a list of "All known pages." This page shows the proportion of your pages that are "Valid with warnings," "Errors," and "Valid with errors."