How do I know if the car is misfiring? I sat behind the exhaust today while the car was idling and I think I can hear some kind of sputtering but I might be imagining it entirely.

If it's misfiring is it always guaranteed to have a Check Engine Light? Because I have none.

I thought my head gasket was blown too because I could have sworn I saw white smoke coming out of the exhaust but I heard the smoke lingers (my smoke didn't) and now I don't see any color smoke at all. The radiator fluid / coolant isn't bubbling either so I guess I'm in the clear?

 

Generally speaking - not knowing what the problem/car/engine/mileage are - you did good listening the tail pipe, I do the same.
You may get a misfire once-in-a-while, however not consistent enough to set a fault - engine light.
How's the car run?