How do I know the difference between swelling and fat? (Photo)

Mainly by time passing. The more time passes, the less likely there is significant swelling. Certainly by 4-6 months the vast majority of noticeable swelling would be gone. Remember that you will still have fat left. Not only because it is normal, but in Florida only 1000cc of fat can be removed at the same time as your tummy tuck if it was done in an office/clinic.

There are several possibilities that might explain your swelling.  It's entirely normal to have swelling in the front lower part of your abdomen for months following surgery. Often this swelling is worse at night than in the morning and eventually disappears.  A different type of swelling is called seroma which is a fluid collection that might need to be relieved. See your plastic surgeon who will know what it is and what to do.