Forum:PDA help

I am trying to design a PDA for language F = { a^(n)b^(2n) | n Î N1 }

But i don't know how to check that my input string is finished. In fact at the end of my input string I need to check the status of my Stack. If it's empty or not. ... please tell me how to do that in JFLAP??? I am unable to check that my input string is finished. :(