For those it might help, considering that I do not know if there are any side effects when using the eZHTTPTool as I do (sounds strange that these operators are not natively proposed by eZ Publish).
I created an operator to write session variables :
You should return null value if session is not set, so you can figure out if session is set or not.
But I think there are several contributed extensions around that does this (and the exact same way). If you wrap this up in an extension, remember to label it with a big warning sign about using it in combination with node templates( hint: view cache ).