How can I tell if a serial connection is sending bytes okay?
If you are using the auto-baud rate setup for Pololu motor systems you first need to send "a baud rate indication byte", basically you send an 0xAA first, and after any power on or reset. See descriptions of serial settings here: https://www.pololu.com/docs/0J44/6.1  

It does not seem to say if the "indication byte" is acceptable with every transmission.  Overall it may be best to just set the system up to a known baud rate ahead of time, or use the default (which seems to be 9600). 

To physically verify transmission from your Teensy controller you could also put an LED and resistor on the transmit line (say with a 500 ohm resistor to gnd or V+).  The LED should flash briefly with each transmission packet, though at higher baud rates the flashes will be quite short, (so if that is a problem do some testing at lower rates for verification).