You can use the
VISA Bytes at Serial Port property to check the number of bytes in the input buffer of the specified serial port.
You can create a VISA Property Node from the function palette by selecting
Instrument I/O >> VISA >> Advanced >> VISA Property Node. Click on the property, then select
Serial Settings >> Number of Bytes at Serial Port.
Once the number of bytes at the serial port is greater than zero, it indicates that there is incoming data in the input buffer of that respective serial port and it is ready to be read.