A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Jason is on the right track but for my conditional formatting formula I used
=ISNUMBER(FIND("!",FORMULATEXT(A1)))
In the screenshot two cells (C1 and C3) reference another sheet;
I selected all the cells and used the formula above (note it refers to A1 since this was the first cell in my selection
best wishes