It makes a distinct sound when it crosses a threshold (Destiny)

by General Vagueness , The Vault of Sass, Tuesday, May 19, 2015, 20:42 (3720 days ago) @ stabbim

I experimented with this a little more and I think Vaguness is right. There seemed to be three distinct tiers of rising beeps, and then it just holds at the last tier. That last tier is where I was seeing damage numbers like what ZackDark described.

There's a visual cue too, the rocket (or whatever you want to call it) glows brighter after each set of rising beeps.
What's unexpected and kind of impressive to me is the enemies seem to know what's going on-- after the first or second set of beeps, most of them will hunch over and stop moving, like they're trying to protect their allies from the explosion they know is coming.