I've mage a gizmo to help compositor to visualy checking the technical quality of their comp in the final stage.

Features, you van check :

-  if the grain is matching the plate and moreover if there is any area without grain.

Those areas will show up without smooth and dark, while the grained area will appear very noisy.

-  the overall luminance to check your black and white level.

-  the saturatrion levels in your comp. This is very handy to see if the black level of your cg layers are matching the one from the background plate.

- the illegal values : negative values and super highlights.