You can use terminal.print() but then you need terminal.flush() so I simply do a virtualWrite() to terminal for any text I need to see.
I would suspect that it is not working the way you think… the sketch is probably ignoring the V in this case and reading the state of pin 3 , which on a Wemos D1 Mini happens to be the RX pin (don’t know what it would be on an old D1 UNO shaped board).
For the benefit of any one doing a search in the future - the fault was in the wemos driver shield’s firmware.
If you’re going to use one of these you may need to flash it with a more reliable open source version available here: https://hackaday.io/project/18439-motor-shield-reprogramming
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You called it.
“When you eliminate the impossible whatever remains, however unlikely, must be the truth”
@Taciturn,
There were several issues with this Shield (at least at the very beginning), mine is not able to run after a 10 seconds delay, it’s really disappointed…
I heard time ago there were other shields without firmware for the STM32F030 microcontroller… At least the one I have works with this limitation! 
I saw some compliants about the drivers but (foolishly) thought no one would continue to sell a board with drivers that were known to be defective.
But it turns out they are. The example sketches work because they run at boot and send continual commands- giving the impression the board is working normally. But as you have seen the motors can’t be controlled beyond that.
The firmware update I posted the link to above seems to have resolved my issues, I already have another board in its way but at least I know how to fix it too if I need it for another project
Good to know you solved your issue, could you please be so Kind to do a small test for me regarding the 10 seconds delay? If this problem is solved also, I would try to update the firmware.