Quality lactation centers (like "Day One" in SF) will rent you a digital scale.
Weight baby before and after breastfeeding, and keep a log. But don't change her diaper in before the 2nd weighing for a feeding - cause the digital scale picks up that ounce! If baby is gaining weight, everything is good. If you notice over a week baby is only loosing weight....
(if she was only bottle fed, we could say a 9lb baby needs 24 oz a day but that math is out the window when she is on the breast. On the breast is best for your supply, hence the digital scale)
Or call your doctor for a perscription of Reglan, explain you are going back to work and need the support to keep your supply up.