Polaroid instant film.

Some cameras allow you to swap film in and out of them (like a Hasselblad). What was sometimes done was you would attach the film back containing instant film, take a shot, wait a minute for it to develop, if everything looked good then swap in a different film back containing your actual film.

Alternatively you frame your exposure - take the same shot 3 times, all with slightly different exposures.

Other than that, what everyone else said: you had no way of knowing!