Validate if value is present in list or not on click of a button

[pratiekk]

I have a list with a Name column. A form with text box and a button.
On click of a button on the form, I need to validate that text entered by the user in textbox is already present in the list or not.
If yes, then message to be displayed that the item is already present in the list. 

[emha]

does it have to be on a button click?

with calculated value and lookup() function you could perform the check instantly, everytime the value is changed.