I have a Buffalo WHR-HP-G54 router which has a SoC with UART support and 4 pins on the PCB. I easily detected the GND. According to this:
The VCC should be fixed at 3.3V and connected to the supply layer ; the TX is also at 3.3V level, but using a multimeter as an ohm-meter, if you find an infinite resistance between the TX and VCC pins, it means they're distinct signals (else, you'll need to find the correct TX or VCC pin). The RX and GND are by default at 0V, so you can check them using the same technique.
..voltage between GND and Rx pin should be 0V. If the system is powered on, voltage between GND and all other three pins is 3.3V.. In addition, is this always true that resistance is infinite between Tx and VCC pins? Last but not least, why is VCC signal needed?