Alert of a number entered on sheet 1 already existing on sheet 2.

On sheet 1 of an Excel spreadsheet I maintain a log of engineering drawings in need of revision. Certain drawings require that the customer is notified before the revision is implemented, and those particular drawing numbers are listed on sheet 2. I would like to be alerted of a drawing number requiring customer notification when I enter it on sheet 1. In effect, when a drawing number is entered in a cell on sheet 1 a search would occur on sheet 2 for the number entered. If it is not found on sheet 2 the number is entered normally. If it is found, the text "notify customer" would be returned in the sheet 1 cell.

Does anyone know if this can be done, and if so can you provide me with the formula needed to do it?

I am using Excel 2002.

You can't do it with a formula, as when you type into the cell on sheet 1 it would overwrite any formula you had already put in.

Probably the best thing would be to put in the cell next to where you enter the drawing something like this in B2 on sheet 1:
=IF(OR(COUNTIF(Sheet2!A:A,A2)=0,A2=""),"","Notify Customer")
Where, A:A is the column with the drawing numbers and A2 is the cell with the drawing number you have entered.

Thanks a lot yudlugar! That worked perfectly.