validation for excel sheet

For that you can do 2 things.

1.Do client side validation for excel sheet.i.e.by using cell formatting,formula,etc.

2.Do server side validation for the uploading data by using reg-expression or other validation methods

As a example how to check empty string ?

private string EmptyStringValidation(string strValue, string strFieldName)
        {
            if (string.IsNullOrEmpty(strValue))
            {
                return "Empty " + strFieldName + ", ";
            }
            else
            {
                return string.Empty;
            }

        }

I hope this will help to you.