Transmitting, then we generally don't check in the same way and just tend to get away with it, unless we start exceeding the TX buffer that
Huh? Serial.write() (and everything that calls it) is a blocking call. If you send more data than fits in the output buffer, the call just waits until there IS room in the buffer. (Serial.read() is non-blocking, which is inconsistent, and causes many users some confusion, but if you're going to be inconsistent, this is the correct way to do things.) (Note that the size of the serial output buffer in Sparkfun on Adafruit SAMD boards is effectively ONE byte!)