You should be able to do a basic test to see if the RS485 Pi is transmitting using a digital multimeter and the program minicom.
First set your multimeter to measure DC voltage and connect your probes to the A and ground pins on the RS485 output pads. You should measure a voltage of approximately 1.3V.
Repeat the measurement between the B and ground pins, this should also show 1.3V.
A voltage of 1.3V on the A and B pins will indicate that the transceiver on the RS485 Pi is generating the correct voltages.
If your multimeter can measure in the millivolt range, 0 to 20mV you can check if the RS485 Pi is sending data by setting your multimeter to measure AC voltage, connecting your probes between the A and B pin and use a terminal program like minicom to send data through the RS485 bus.
Start mincom with the device set to /dev/ttyAMA0 and the baud rate at 115200.
sudo minicom -b 115200 -D /dev/ttyAMA0
With the multimeter connected to the A and B pins it should show a voltage of 0V.
Next press and hold down a key on your keyboard so minicom keeps sending the same keypress down the RS485 bus. The multimeter should show a small voltage when the key is pressed. On my multimeter it showed between 17 and 22mV depending on which key I pressed but your multimeter may show a different voltage depending on how it samples AC voltages.
If you get any voltage change that will indicate that the RS485 Pi is sending data down the bus. To do more advanced tests you will need some more advanced test equipment like an oscilloscope or logic analyser to probe the RS485 bus or a RS485 device to act as a receiver so you can try sending data between the two devices. You could use either a USB RS485 transceiver connected to your PC or a second RS485 Pi on another Raspberry Pi,