Try IsNumber(). It returns a true or false. You can put this inside an if
to produce other messages, for example: =if(isnumber(A2),"","OOPS").
Alternatively you can use a conditional format within the cell you're
testing to change the format (like background color) if it's non numeric.
To do this, go to conditional format, select "formula is", and type:
= NOT(ISNUMBER(A2))
And then select your format
Art