Transmission cooler PLUS radiator?

Maybe I should clarify. How do I distinguish the flow direction from the transmission? Because I'm going to want to hook up the inlet to the radiator, crank the car, and let the first 10-15 second of trans fluid that comes out of the radiator to drain into a pan. I figure the first stuff through will be finish cleaning the transmission lines in the radiator, and I'd rather that not go back into the radiator...