Led must be mounted SINK or SOURCE so as not to interfere with communications?
RX and TX lines at TTL signals** between the μC and the line driver IC*** is a good place to connect the LED indicators.  There's no good reason to connect the LEDs to RS-232 lines.  RS-485 lines are bidirectional, so the LEDs alone wouldn't tell if the data is TX or RX on RS485.

** 0 to 3.3V serial
*** RS-232 or RS-485 line driver, and such

Back to TTL signals.  You can connect the LED between signals and GND **, or between signal and Vcc ***.  Either would work as long as the LED current is low enough (limited by a series resistor).  LEDs add some capacitance, which might be a problem at high speeds above, say, 1M bits per second.

** source in the O.P. terminology
*** sink