Using Vivado's Virtual I/O IP block.

[hamster]

One thing I learnt today was that Vivado has a Virtual I/O IP block, that allows you to see the state of signals deep in your design. You can also use it to inject signals into your design too. 

I've added a little project to my Wiki which connects the switches and LEDs on the Basys3 to an instance of the Virtual I/O block. 

http://hamsterworks.co.nz/mediawiki/index.php/Virtual_IO

(Connecting to external I/O is not really the target end-use case -  it would be perfect for monitoring status signals out of transceivers and memory controllers, or observing the state of misbehaving FSMs)