One of the easiest ways to tell if the image is underexposed is to add a grey card with a value of .125 diffuse in front of the camera. If it reads as 99,99,99 in PS that its exposed 'correctly'
Agreed with @PixelMasher a lighting overhaul could really finish this. Boosting both exposure would be great EG Setting bounces to 10+ would help, and pushing diffuse boost beyond 1 would really work with this art direction, allowing the bounce light to get pretty saturated.