Detecting values in GET Http header

Hi. I have a small but annoying problem.
I'm using HTML with GET. In the page there are:
1.Combo box in which : 1.1. a value is always selected
1.2 'On change' event causes the form to
submit
2. Some submit buttons
The problem is:
After the form is posted to the server, I do not know if it was submitted
because that the user changed the value in the combo box, or beacuse he clicked one of the submit buttons.
( the GET ALWAYS contains the value selected in the selected box)
Possible Solutions:
1. On combo box's ON change event -> submit a hidden button,
and then check on the server if this button name was sent to the server.
( but I was able to do it with type="Submit" NOT with type="button"
So, as a result the 'hidden' button appears on the page!!!
2. Using CSS to hide the button doesn't work as well.

That's all, if you have any solutions id'e love to hear it
thanks
roy

It is simple, but I couldn't saee it before...
<input type="submit" name="b2" id="b2" style="display: none">