C# - Validate form items

I'm working on a C# page that has a lot of textboxes, drop downs, text areas.

Last Name:
First Name:
Contract #:
Phone:
Email:
Birth date:
Location:
Start Date:
End Date:
Salary:

......

The data will be stored in 3 different tables
EMPLOYEE (EmplID, Last Name, First Name, Phone, Email, Birthdate...)
EMPL_LOC (EmplID, ContractNbr, Location...)
EMPL_SAL (EmplID, StartDate, EndDate, Salary...)

Items are not in order.  #1 (LastName),#2 (FirstName),#4,#5,#6 - Table EMPLOYEE, #3, #7 - Table EMPL_LOC...

Everytime the user clicks on Update button, I want to check to see which item(s) got changed to update appropriate table(s).  For example, if the user only changes "location", I just want to update only EMPL_LOC table.

On PageLoad, I can get all the values on the form

formcount = document.forms(0).length;
            for (i = 0; i < formcount - 1; i++) {
                itemType = document.forms(0).item(i).type;
                switch (itemType) {
                           case "text":
                                          iALL_Items_Values = iALL_Items_Values + document.forms(0).item(i).value.toString().trim();
                                         break;
                           case "select":
                                          iALL_Items_Values = iALL_Items_Values + document.forms(0).item(i).value.toString().trim();
                                         break;
                           case "radio":
                                          iALL_Items_Values = iALL_Items_Values + document.forms(0).item(i).value.toString().trim();
                                         break;
                           case "checkbox":
                                          iALL_Items_Values = iALL_Items_Values + document.forms(0).item(i).value.toString().trim();
                                         break;
}
}                        

When the user clicks on the Update button, I call the function above again to get the values on the screen and compare to the values that I captured on PageLoad to see if there's any changes.

How do I do it for each table, only check #1,#2,#4,#5,#6... to see if table EMPLOYEE needs to be updated, #3,#7... to see if EMPL_LOC needs to be update, and so on...

Thank you very much for your help.