Comments tell me my initial assumption was incorrect, but I’ll leave it here, just in case.
You cannot use the rear screen on many cameras to preview exposure, as they auto-adjust to try to give a ‘best view’ in all lighting.
I’m assuming your softbox flash was a regular strobe light?
That’s why the pros use studio flashes, which contain modelling lights. These let you see the general shape of the lighting before the shot is taken, and is also hugely useful to just be able to actually see in the room, and as something to help out auto-focus.
The idea is that the flashes almost completely overpower the modelling lights, so they don’t really interfere with the end result.
Being in a similar position, I only have regular strobes, I often use video lighting squeezed in tight to the softboxes to mimic modelling light. It doesn’t quite work, because you cannot achieve the exact same angles, but it’s a reasonable close-approximation, for zero additional budget [as I have them anyway, for video].
I have considered trying to squeeze some small consumer LED spotlights, rear-facing inside the softboxes to better approximate my angles – but this is a project currently awaiting trial.