I would like to create a conditional format based on the content of the cell,
rather than the value.

I have a spreadsheet that has empty cells, cells with values of 1 and 2, and
cells that will have values of 0 based on 1-1 or 2-2. I do not want to
highlight the blank cells, I only want to highlight the cells that contain a
formula, even if the formula's value is 0. How can I do that? "Cell Value"
looks at the result of the formula and highlights all blank cells. I cannot
figure out what formula to use to look at the cell content rather than the
cell value.

For example:

Cell A1 is blank -- no highlight
Cell A2 value is 1 -- no highlight
Cell A3 value is 0 because formula is =1-1 -- highlight.

I only want to highlight the last cell because is contains a formula.

Thanks in advance!