You could T in a vacuum gauge on the bottom end where the vacuum pump will be connected to the lower side. A mechanical version may work, but not sure being the atmosphere will likely be heavy with oil vapor and mist. Another option would be to obtain an electrical vacuum gauge but you're looking at $250 for the only model I could find. A vacuum boost gauge that reads vacuum down to 30inches would also work. They are a bit more reasonable as they are in higher demand with regard to one with a sending unit vice the mechanical option if you have to go that route.