Heat means resistance. I would use a voltmeter and check the voltage at different points along that positive line, when it's under load. Connect the voltmeter negative (black) lead to a good ground, then touch the positive (red) lead to each terminal and each connector at each terminal. If you do this while there's a good load being pulled, then you may find a lower voltage at one point...and this will happen right "after" (downstream of) the resistance point.
Another, more sensitive, way to do this is to use both leads of the voltmeter from point-to-point on the positive cable terminals and connectors. In other words, touch one lead to a terminal's top and the other lead to the terminal's metal. If the connection is good between them, there should be almost 0 voltage reading. Resistance will show up as a voltage reading, meaning a differential in voltage between those two points. This test should also be done with the line under load.