Determine direction of RS485 communication
Use current probe. There is a 120R resistor on each side, but only one side drives at a time. So with current probe you can know the direction. Typically the current would be around 25mA, visible enough. 

Actually its best to use current and voltage probe together. So when voltage on a wire is positive relatively to the other wire, current poits to receiver. Otherwise it points to transmitter. 

By the way. If current probe is not available, just cut the wire, insert a 10R resistor and measure voltage on it.