There is no rule of thumb, it tends to come naturally after a while.
Saying that for formal portraits they used to use polaroid proofs in the film days to check exposure and dof.
In the digital age you already have a built in proofer in your LCD screen, take the shot magnify it to check if the subject/s are in focus and background out of focus and adjust your aperture if and when as needed to get the result you want.
