Originally reported by @TheJeffr0 at https://core.trac.wordpress.org/ticket/64165

In the following screenshot, a number of these blocks have Notes attached to them. But you can't tell. There are a handful of notes in the panel, but there's no way to tell which one belongs to which block without clicking on them, which may be the way it's supposed to work.

However, figuring out which blocks have comments and which don't requires the user to click on all of them. Even if the Visual Indicator is the Small avatars in the block's toolbar, it doesn't help distinguish, at a glance, which blocks have notes and which don't.

Can't tell the difference between blocks with notes and blocks without them

This is great feedback @jeffr0 - thanks for the bug report.

We discussed adding icons over the blocks indicating the notes as one possible solution. If you go into the "floating" mode by closing the Notes panel, the notes float alongside their related blocks which makes it easier to tell which blocks have comments.

Hovering over notes does highlight their related block, but this doesn't really address your question "Which blocks have comments"?