Removing a custom field

Hi I created 3 customs field and played around with them on 3 or four invoices. I then found a better way to do what I wanted and decided to delete the custom fields. two of the three deleted ok, but the third won’t saying it’s in use. I checked the three or four invoices I played around with but still can’t delete the custom field. How do I determine which invoice is the problem?

There is no way other than to look. You should also be sure you have not entered default text for a custom field. Remember, one of Manager’s design principles is to never let something be deleted than might be useful.