How to capture autofill?

[Dev]

I have a value in a cell (A1B1). I am doing autofill (highlight on the cell
and move the cursor to lower right corner of the cell until you see the
cursor becomes a solid black plus sign, then extend the cell to lower cell
(excel way of making a copy of the cell)). Then the value in the cell A1B1 is
copied to the extended cells. How can I capture this event or is there a way
to figure out this in SheetChange event?

Any help is really appreciated!!

Thanks!!