7D remote flash, enough light ?

When using the 580 strobe unit on camera, if I recall correctly,
a light comes on after a flash if enough light was produced for the
automatically determined exposure requirement.

But when using the 7D as a controller, I do not know how to confirm
that enough light was produced, either on the camera, or in the EXIF
data after the fact.

Any idea how I can do this & know if I need to open my lens, or
perhaps change batteries ?