Put a dozen white bbs of equivalent weight into the magazine before you load the remainder with your black bbs. Once you start seeing white bbs, just reload your rifle.

That's pretty genius, I must say.

But what's been said is pretty much it; It's got a pretty distinctive sound, especially an AEG. When it's empty, it'll sound hollower, and more like a 'thwack' than a 'whwump'.

There's not really much we can say to help you (Everyone's rifle is different), so my suggestion is to do that ^^^ or simply do a couple test fires at home where you load in a few rounds, listen to that sound, then fire it empty, and listen to that sound.

Not much else, except that when you get familiar with your rifle, it gets easier to notice.

Works the same if you have a tracer unit.

You could always be a total dork and geek load. When I ran .25g, I put in 10 white, then 5 black, 1 white, 5 black, 1 white until it was full.