check if user typed in asp textbox

I'm using ASP.NET Web Forms.

In a ASP TextBox that holds a string, how do i check if a user typed in the textbox?
Sometimes the textbox might contain a string already on page load. I'm not checking if the textbox is empty.
I want to check if the user typed anything in the textbox.

So on button click if they user typed anything in the textbox

Then set this variable CheckTextBoxTyped = "Yes"

If nothing was typed in the texbox set this variable CheckTextBoxTyped = "No"

Right now i have this:

    Protected Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

    Dim CheckTextBoxTyped As String

    CheckTextBoxTyped = "Yes"

    CheckTextBoxTyped = "No"

    End Sub
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Paul MacDonaldDirector, Information SystemsCommented:
On Page_Load, capture the value of the textbox.  When the user submits the page, see if the value of the textbox is the same as the stored value.  If they're the same, you can presume the user typed nothing.  If they're different, you know they did.

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