How to tell if an air-separator is working
The maintenance and operation documentation, for what I think is your model, provides the following steps if service is required.  


  WARNING! System fluid under pressure can be very hazardous. Before removing the vent head for inspection, reduce system pressure to zero,
  or isolate the Spirovent from the system. Allow the system to cool to
  below 100°F. Failure to follow these instructions may cause serious
  bodily injury or property damage. 
  
  
  Using two strap wrenches, hold the body in place in the piping while
  removing the vent head, taking care not to damage the O-ring. Pipe
  wrenches may be used, but they can leave marks in the brass.
  To clean the vent head, use a mild detergent and water or vinegar
  and water solution. For extremely dirty conditions, soak the head in
  mineral spirits for several hours and then clean thoroughly. The
  vent head is available as a replacement part if necessary. Use part
  number PJR000HA. Assembly includes the O-ring.
  While the vent head is removed, remove the coalescing medium from
  the body and rinse thoroughly with mild detergent and water or
  vinegar and water to remove any system contaminants.
  Examine O-ring before installing on the Spirovent and replace if
  necessary. Use part number PJR000RG.
  Reassemble the Spirovent.
  Return system to operating condition.
  


It's a bit of work, but I think it will be the most complete way to ensure the vent is in good condition.