you won't need a multimeter ever again
Identify all UART pins by flashing a simple script.
- Flash the script to an Uno.
- Connect the UART Pins to A0,A1,A2,A3 in any order.
- Switch on the device.
- Connect the Arduino to your PC.
- Open up the Serial Monitor.
- See the magic happen.
This has been tested on a couple of routers and on an ESP8266 with 100% success rate.