When I installed an auxiliary transmission cooler in my '95 (25 or so years ago, now), the way I found out which was which was to crank it up and feel both lines. The line that heated up first was the output line or the line going TO the transmission cooler. If you do not have a transmission in the car or it won't run, you may have to do some diggin'. I'm sure someone in here knows right away which is which. Sorry if no help.