We sell a bunch of Nitrogen Purge Alarms, but they are all for. high pressure cylinders that will alarm when the pressure drops to 400-500 psi.  In a liquid dewars, the pressure won't be that high and won't drop until it's empty.  Weighing the dewar is probably the only way, but how do we comply with NFPA-99?

Al is correct there is no mandatory device (alarm) attached to a purge regulator, so long as there is someone monitoring the cylinder contents.  However, there are adjustable alarms capable of monitoring a liquid cylinder's pressure.  The ones I have seen in the field are white and look dome-shaped.  If you are looking for one, do a Google (images) search for nitrogen purge alarm.  Two out of the first pictures shown are the ones I am familiar with...