How to find how much data in the transmit buffer?
If you really want to know (not that it would do you any good) you can use the Serial.availableForWrite() function (which returns how much space is left in the buffer) and subtract the returned value from the size of the buffer.

int used = 64 - Serial.availableForWrite(); // Assuming a 64 byte buffer


I haven't tested it, but you may be able to use SERIAL_TX_BUFFER_SIZE instead of the 64 to make it properly portable.

int used = SERIAL_TX_BUFFER_SIZE - Serial.availableForWrite(); // Possibly portable