Pump the cold furnace down as low as possible, close the vacuum valve and time the rise in pressure from that point to another.  Calculate the rise in pressure from point A to B to get a millitorr (micron) per hour amount.  This will give you what is called the rate of rise.  If the rate of rise is to high then do the following:

Connect a helium mass spectrometer into the pumping line in front of the vacuum pumps. Reduce the pressure as low as possible and begin to spray helium around all penetrations of the vacuum vessel starting at the top, as helium rises.  Care must be given to the maximum operating pressure of the spectrometer relative to the pressure of the furnace.