How to Differentiate Between a Seroma and an Untreated Pocket of Fat After Vaser Lipo?

Swelling after #Vaser liposuction is not uncommon. Gentle #walking may increase circulation and help to alleviate swelling. But you must not strain or push yourself. Also, if you were provided a post-procedure #garment, be sure you are wearing it until prescribed otherwise by your doctor. The purpose of the post-procedure #garment is to provide some gentle #support and to reduce swelling. If it feels too tight or causes pain, take it off. Many patients wear the #elastic garment for several weeks after procedures, as directed by their doctor; some choose to wear it longer for several months for comfort.
A #seroma is a collection of fluid under the skin occurs occasionally during the postoperative period. Such would better be confirmed with an in-person visit.  Depending on the size and condition of the #seromas, aspiration of the fluid with a needle is frequently helpful but not always necessary. Secondary surgery is rarely required.
The final results of the procedure are typically most noticeable 2-4 months following the #Vaser. So, you must have patience. The swelling will gradually diminish while the contours of the treated area(s) can continue to improve up to one year post procedure; becoming more apparent in appearance. If you have concerns about your healing, or pain that you question to be unusual, it is important to call your plastic surgeon to discuss these further asked to be examined. 

The most objective way is to get an ultrasound to see if it is fluid (seroma) or fat. Fluid has a different sonographic appearance and can even be aspirated by needle by the radiographer if appropriate (ie depends on the size and amount of seroma).

It's difficult to say without a physical assessment of your treated area.  Find a plastic surgeon locally and schedule a follow up immediately.  If it's fluid it will need to be drained and looked after by a surgeon.