Thanks Joe4. However, for various reasons (mainly vastly superior interface IMO) I'm using Excel for Mac 2011, and while Microsoft's website says that FORMULATEXT works in XL Mac 2011, I've tested it, and unfortunately it doesn't: it returns a NAME? error. So I'm out of luck with that.
Jonmo1, your link was a gold mine! I'm an old-timer (started with Excel in 1986, before Excel for Windows even existed) so I'm well aware of the old XL Macro language: in fact, use it all the time in preference to VBA. The solution in the link you referred to looked like it would be hard to use: in essence, you have to define a function using the old GET.CELL function, but in the absence of an OFFSET, the defined function is trivial, because it always returns TRUE for any cell you use it in.
Very very fortunately, I didn't need to use my CellContainsFormula function in the cell itself, but in a Conditional Formatting formula. My overall objective was to use a single CF rule for hundreds of cells: each would be shaded green if it contained a constant instead of a formula. And this worked magnificently!!
For the benefit of others –
1. In Insert > Name > Define... I defined the name "CellHasFormula" to be "=GET.CELL(48,INDIRECT("RC",FALSE))".
2. I created a Conditional Formatting rule that used the following formula to determine whether to apply the shading: "=NOT(CellHasFormula)", and applied that CF rule to hundreds of cells.
3. Now, every time a user replaces the existing formula in any of those cells with a constant, the cell is highlighted!
Victory! I'm stoked!! Thanks Jonmo1 and PaddyD!!!