How Can you Tell that Fat has Actually Survived the Transfer? Biopsies?

There have been several studies looking into fat graft survival. Quite recent MRI studies have shown the persistence of grafted fat in the face. The take isn't 100%, but using proper placement volumes reliable results can be obtained. Certainly, a secondary touch up procedure may be indicated after an initial fat transfer has fully healed (in up to 18 months).

One way to confirm that fat has survived is through photos. Look at before and after images up to twelve months after surgery.For transferred fat to truly qualify as a 'graft' the following must happen: living tissue must be transferred to a new location, and that tissue must gain a blood supply at the new location which provides oxygen and nutrients which allow it to persist indefinitely as living tissue.  We know that with appropriate instrumentation and technique this is achievable, so one of the opinions expressed in this thread that "most of the cells from fat injection are dead " is simply untrue (and structural fat grafting, to be clear, does not involve 'injection' of fat).  MRI studies have shown that with appropriate technique grafted fat persists long-term as living, vascularized tissue in the recipient site
There are many fat grafting before and after photos on websites of plastic surgeons who regularly perform fat grafting, including my own that can show results of fat grafting after many months and years.