There isn't an easy way to test the RS485 Pi directly other than to connect the outputs to an oscilloscope and see if the signal changes when you send data.
You can test the UART port on the Raspberry Pi, to make sure it is sending data to the RS485 Pi, by connecting the TXD and RXD pins together on the GPIO header and use a terminal program such as minicom to send characters out on the TXD pin. If the UART port is working anything you type should be echoed back into the RXD input and display on the screen.
TXD is pin 8 and RXD is pin 10 on the GPIO header.
If you don't get any characters appear on the minicom screen when you type then there is a problem with the UART configuration and I would recommend going through the steps in our serial port tutorials. For the Raspberry Pi 3, 3+ and Zero W you will need this tutorial, for all other versions of the Raspberry Pi use this tutorial.