How do Plastic Surgeons differentiate between parotid leak and seroma?

The answer to your question is that given the location of the fluid accumulation, the surest way to differentiate between a seroma and a parotid fistua is to send the fluid for an amylase test.  This will usually respond to drainage and compression, although sometimes it is helpful to treat with botox as well to reduce the rate of secretion.

To tell the difference between parotid gland secretions and a seroma  have the fluid analyzed by doing  an amylase test. Amylase is an enzyme found in saliva  produced by the parotid gland.  A Scopolamine  Patch, Botox, and staying away from spicy and sour foods are often effective  to resolve fluid collection from the parotid gland (sialocele/fistula) originating from the parotid gland.

 Location may be an important factor determining a seroma versus a parotid leak. An amylase test can be helpful but simple clinical observation of the area before and after eating can tell you if this is a parotid leak. The area will increase with eating because saliva is used to help digest food.  The leaking area will leaks allowed into the space in your face that being treated and this will most likely tell you that this is a parotid leak versus the seroma.