Years ago I worked in-house at a startup magazine called, if I’m remembering correctly, Real. The publication itself didn’t last very long, but my time there was worth it for one takeaway: a piece of advanced comma usage advice the showrunner at Real imparted to me that I’d never heard expressed before, and haven’t heard anyone express since. It occurs to me now that I can use this space to pass that wisdom along. This rule of thumb isn’t applicable in every single case, and there will always be exceptions and judgment calls, but it’s served me well. It has to do with whether you use a comma before too (or either or as well or similar).

If the word you’re referencing does not immediately precede too, no comma is necessary:

In the example above, this would mean that the titled nobility, in addition to someone else (perhaps the royal family?), were relieved of their heads.

Compare that to the following:

If you want to emphasize that the revolutionary heroes executed her, in addition to someone else, you use the comma before too. If you were making some odd point about how they both captured and guillotined her, you’d leave out the comma because captured and guillotined come earlier in the sentence (though that would be an awkward way to put it).

Again, this won’t work every time you see or use too, but thinking about which other word in the sentence is being referenced can be a starting point when you’re trying to figure out whether you need the comma or not.