Or you can try this....
Get behind another vehicle (preferably one emmitting lots of smoke and fumes) with the recirc. "on." If you can smell the fumes, it's not working.
No two ways about it.
(And if it isn't, don't forget to get away from behind that car immediately!!!)
By the way, air recirculation won't work if you engage the A/C economy mode (that's if your vehicle has that feature).