Conditional formatting (CF) in Excel can be used to hightlight cells that meet certain criteria. In this case, I wanted to create a CF that would highlight cells containing formulas that use Excel’s native 3D references. So, this would be like the following example.
=SUM(Sheet1:Sheet2!B2:B5)
So, I tried to determine what was unique this type of formula string compared to others. What I noticed was that the first colon in this formula always comes before the exclamation point. Thus, I started working on a solution on that basis.
Note, though, that there are ways to write a formula containing a 3D reference that will not meet this criteria, such as:
=SUM(C2:C5,Sheet2!C2:C5)
So, don’t use those kinds of formulas. 
To lookup the position of the colon in the formula string, the following formula is needed.
=MATCH(“:”,MID(FORMULATEXT(F6),ROW(INDIRECT(“1:”&LEN(FORMULATEXT(F6)))),1),0)
where F6 contains the formula.
The corresponding formula for looking up the position of the exclamation point is:
=MATCH(“!”,MID(FORMULATEXT(F6),ROW(INDIRECT(“1:”&LEN(FORMULATEXT(F6)))),1),0)
By comparing the two formulas, the following Boolean expression wrapped in an IFERROR function is defined as Is3D:
=IFERROR(MATCH(“:”,MID(FORMULATEXT(F6),ROW(INDIRECT(“1:”&LEN(FORMULATEXT(F6)))),1),0)<=MATCH(“!”,MID(FORMULATEXT(F6),ROW(INDIRECT(“1:”&LEN(FORMULATEXT(F6)))),1),0),FALSE)
Applying this formula as a CF on cell F6, you can see that F6 is highlighted as expected.
You can download the example file here.