How to check the value of lookup is selected or not

How to check the value of lookup is selected or not

I have one lookup in which i have names and after selecting names i am displaying the address of the selected name.but if the lookup value is not selected then i want to display the text-boxes to accept data from user.I have "-Select-" field at top if user selects that "-Select-" then i want to display the text-boxes to accept input from user.
how can i do that.