If your data is all numeric and is formatted to show negatives using minus
signs, maybe you could look for -.

If both those things aren't true, then I think you'll have to loop through the
cells.

You may be able to reduce the cells to loop through by looking at just the
numeric cells in column D.

If your data is all constants (no formulas):

dim myRng as range
dim myCell as range

set myrng = nothing
on error resume next
set myrng = worksheets("sheet999").range("d:d") _
    .cells.specialcells(xlcelltypeconstants, xlnumbers)
on error goto 0

if myrng is nothing then
 msgbox "No numbers!
else
 for each mycell in myrng.cells
    if mycell.value < 0 then
       'found it, do what you want
    end if
 next mycell
end if

(Untested, uncompiled.  Watch for typos.)