Information Gathering

Detect UART Pin

Check the board

some hardware mark the UART pin (if we super lucky).

Use Multimeter

Ground

set multimeter to continuity mode, short the multimeter to hear the Beeb.

Put the ground (multimeter) to the plat or any ground on the device. Then test any pin. If you hear the Beeb, it mean that is the ground.

VCC

set multimeter to 20V, then put the ground (multimeter) on the ground device. and test the others pin.

If its return 3.3 V constantly It mean VCC.

If it always changing the volt it mean RX

If it constant volt, it mean TX.