There are four transmission mode definitions and elaborations for RS-485, as explained in Transceiver Modes on NI RS-485 Serial Cards. The four-wire transceiver mode is the only mode in RS-485 that supports programmatically setting or clearing RTS signals.

To verify this function, you can perform an "RTS/CTS loop." Since the CTS signal changes with the RTS signal, by connecting the corresponding pins of the RTS and CTS, you can verify that the RTS can indeed be set or cleared by the program. To do this experiment using the RS-485 port and the corresponding cable, simply connect RTS+ to CTS+ (pin 3 to pin 2) and RTS- to CTS- (pin 7 to pin 6). Then, simultaneously set or clear the RTS signal and read the CTS signal. Since the CTS is connected to the RTS, the value of the CTS will be the same as that of the RTS. For the RS-232 interface, there is no need to consider the transceiver mode when performing similar tests. It can be done by connecting the RTS and CTS lines (pin 7 to pin 8). If you need guidance on how to write this test program using the VISA property node in LabVIEW, see the resource How Can I Control the DTR and RTS Serial Lines in LabVIEW? 

According to this document, you will be able to use the property node to read the state of the CTS to verify that it follows the state of the RTS. The only factor to consider for RS-485 is to use a property node to set the mode to four-wire mode. To do this: 
 

1. Place a VISA property node on the block diagram (Functions palette » Instrument I/O » VISA » Advanced).
2. Right click on the attribute area of ​​the attribute node.
3. Select Property » Serial Settings » Wire Mode. As shown in Figure 1.
4. Right click on the property node and select Change to Write.
5. Right click on the property (currently Wire Mode) and select Create » Constant.
6. Select RS485/Wire4 in the newly created constant. As shown in Figure 2.

Figure 1

Figure 2