6 weeks post op tummy tuck with muscle repair and lipo. How do I know if it's swelling or a seroma?

I'm sorry to hear about the concerns you have after tummy tuck surgery.  As you likely know, your plastic surgeon will be your best resource for precise diagnosis, advice, and the most meaningful reassurance...
Generally, abdominal wall "swelling" after tummy tuck may be related to:
1. Swelling in the soft tissues.  This may take several months to resolve and may worsen with increased activity  or at the end of the day.   Patience  is required to allow for resolution of the swelling. The swelling occurs because of the interruption of venous and lymphatic channels that occurs during the tummy tuck operation.
Often patients will like their “flatness” immediately after surgery; then, these same patients are disappointed for a period of time once swelling of the tissues occurs. In the longer term (this may take several months and even up to one year), the swelling dissipates and the skin of the abdominal wall "redrapes"  over the smaller underlying  musculoskeletal framework (where the abdominal wall muscles have been brought together again in the midline). 
Often, patients will report that the swelling increases as their activity level increases and gets worse later on in the day. Sometimes, the use of compressive garments may be helpful to control the edema. The swelling resolves many months (and even beyond one year post op in some cases) as the lymphatic and venous systems begin functioning normally.
2. Fluid accumulation in the space between the skin and the abdominal wall muscle.  this may consist of blood ( hematoma)  or serum (seroma).  This fluid accumulation can generally be diagnosed by physical examination ( occasionally ultrasound may be helpful).  Treatment consists of aspiration;  several episodes of aspiration may be necessary. 
3. Separation of the abdominal wall muscle repair may lead to a swelling/bulge appearance. This may be diagnosed on physical examination  with your surgeon examining you in different bodily positions. One of the steps of a tummy tuck procedure involves reapproximation (plication)  of the rectus muscles.  These muscles have spread apart during pregnancy and/or weight gain. Bringing them together again in the midline helps to “tighten” the abdominal wall as well as to narrow the waistline.
 
4. Residual adipose tissue may be confused for swelling. Again this is most easily diagnosed by physical examination. Additional liposuction surgery maybe necessary to improve the results of surgery  in some cases (this factor does not seem to be likely in your case).
Generally, it takes many months for swelling to resolve after tummy tuck surgery and it may take up to one year (or greater) for complete skin redraping  to occur.
I hope this helps.   Best wishes for an outcome that you will be pleased with longer-term.

It is common and expected to have swelling at this point in recovery. However, if you have concerns or notice changes it is always best to check with your surgeon. If the swelling has been there from the beginning and hasn't changed it is likely typical swelling. If it is new or growing larger it warrants further assessment.

Clinically, an exam would be necessary to sort these things out. Swelling after TT usually lasts 3 to 6 months, but most of the swelling is gone after a few weeks. Healthy people at their ideal weight should have the best overall recovery, but some of this will depend upon the surgical technique and adherence to postoperative protocols.  I always advocate following your PS's post-operative protocols as he/she knows how you are healing and is responsible for your recovery.Kenneth Hughes, MD, ABPS Board Certified Plastic SurgeonLos Angeles, CA