If your PC does not have an RS-232 port (many recent model laptops do not), buy a USB-RS232 converter. It requires a driver for your Windows OS. Get one from an outfit that handles industrial controls, not a cheapie from ebay.
For testing software, buy a Modbus master like "Simply Modbus" or Win-tech's "Modscan 32" or "ModPoll", all can be purchased and downloaded from the 'net.
Configure the comm parameters on both the device and in the software, then try to read a register.
There are a few free utilities to check communications between your PC and your Modbus device.
If your PC does not have a RS232 port (AKA serial port) you may have perchance a USB to rs232 converter, Any Staples or Best Buy will have one.
Once you download and install the utility you will have to configure both the device and your utility to have the same communications settings.
Then it’s as simple as trying to read one of your registers on your device.