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. 

The difference between parotid gland secretions and a seroma can typically be identified through laboratory studies, namely an amylase test.  Amylase is an enzyme found in saliva which is produced by the parotid gland.   There may be situations where the fluid collection may be located near the parotid gland (or by an accessory parotid gland) which may need to be tested to determine the exact nature of the fluid collection.  Many of the treatment strategies are similar for both types of fluid collections, but Botox can be used very effectively to control and help facilitate the resolution of a fluid collection (sialocele/fistula) originating from the parotid gland.I hope this helps.

the only way of differentiating the 2 is with an amylase test. Your surgeon may suspect clinically if the parotid capsule was breeched in surgery but the biochemistry assay of the fluid will give a definitive answer.