You can monitor any Uart of a Microcontroler.
Right-Click on the Microcontrlloler and select "Open Serial Monitor" and you will see a list of available Uarts.
Choose one to open the Serial Monitor widget for that Uart.
You can open several Monitors at the same time.
The serial monitor is divided in two panels:
There is a "Send Text" input box to send ascii characters to the Microcontroller.
Activating the "CR" button will add a return character to the text sent.
And a "Send Value" input box to send 0-255 values.
You can choose to print the data sent/received as decimal Values or it's ASCII representation.

There are "Clear" buttons for each panel at the bottom.