thank you Vitaliy,it'very useful indeed!

A quick and useful way to see how a scene was done is to zoom way in on their eyes. In the fish-eye lens reflection in the eyeball, you can see all the lights that are hitting the front of the face. They tend to be just white shapes though. Gives you a good idea of how big /what direction the source came from, but you'll have to conjecture a bit about how powerful the units were.
Another thing you can do is this: Looking at the light falling on the subjects skin, or even better light that hits multiple subjects, you can get some idea about how close and thus how big the light hitting them was. You already know the inverse square law, but how it works empirically is that the farther away the light is from the subject, the more even the intensity falloff wil be. If you have rapid falloff, you've got a small source relatively close by. If it's even over subjects distributed through a large space, then you've got something huge very far away. Thats why everyone always puts a big HMI like an 18k up in a condor bucket two blocks away- it loses a lot of intensity over that distance, but the difference in brightness between separate objects will be very small, similar to the light coming from that big bright white thing in the sky. Tom Airboxlights.com