I mentioned this once before as an aside, but it is worth a post: a trick that tells you which flash is casting what light in your images.

Say I am lighting a person (like me) with a flash outside. Nice:

Subject lit with an off-camera flash

But how can I be sure this light is from the flash? I mean, is that really all the flash? Or is the subject in the sun? Or in a mix of light?

Solution: put a coloured gel onto the flash. Now you see:

Subject lit with an off-camera gelled flash

Ah. So it was the flash! Not only that – you can see exactly where it is -and importantly, where it is not – illuminating the subject.

Useful trick, eh? One more reason to always carry gels along with you.