assuming LHD, passenger side should be hot from tranny, and drivers side should be cold back to tranny. Check on the tranny itself, the lower line is hot-out, and the upper line is cold-in.
You can verify before installing anything. Take both ends off the radiator, wrap a gallon size clear plastic freezer bag around them and zip the bag on. Put the tranny in park and start it quick and shut it off immediately. Whichever bag has fluid in it, that is your hot from the tranny.