If we are talking about a breadboard one, the scheme I am suggesting would have you use your USB to TTL converter to send debug commands to your PC terminal or to the serial monitor in the Arduino IDE. In your test sketch, using SoftwareSerial, you would transmit a character through the MAX485 and receive it back. You would compare the transmitted to received characters to make sure the RS485 circuit worked, reporting the success or failure through the hardware serial port to the PC.