How to make calculator that allows user to use only 1 decimal point and not more

[NotLevi]

Im making a calculator as a school project and I would like to know how to make it so that only 1 decimal is allowed in the textbox. The current code that need's changing is (And I know its basic but im just learning it):

Private Sub btnNumber_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) _
	Handles btn0.Click, btn1.Click, btn2.Click, btn3.Click, btn4.Click,
	btn5.Click, btn6.Click, btn7.Click, btn8.Click, btn9.Click, btnDot.Click

		Dim btnNumberClicked As Button = sender 'Contains properties of clicked button

		If blnNewOp Then
			Me.txtDisplay.Text = btnNumberClicked.Tag
			blnNewOp = False
		Else
			Me.txtDisplay.Text = Me.txtDisplay.Text & btnNumberClicked.Tag
		End If

	End Sub