Best luck I've had was close up pictures of adults against bright backgrounds, I can't really say either way from these pics, but I also don't have a 100% ID rate when I think I can so.... yeah.
Quickest way? Introduce them to each other. You'll know within about a minute and if you're ready with a trap/net, you should be able to safely remove the bullied one. In my experience, they will be chased to corners/near powerheads rather than into the rockwork or similar, but usually introducing a net nearby makes the aggressor back off in any case.