[Haskell-cafe] how to check thunk

Hello, Are there any ways to see if a value is a thunk or memorized? I would like to have a function like: isThunk :: a -> IO Bool I'm implementing some data structures described in "Purely Functional Data Structures" and want to check my implementation is correct. E.g. https://gist.github.com/2987425 Thanks. --Kazu