How does this YouTube video's author identify pins with multimeter?
VCC will have a higher voltage, txd will be almost as high, ground will be lowest, (and also be connected to other known grounds). and RXD will be somewhere in between.
tx is an output, idle uarts are high, but no transistors are perfect consuctors, so the voltage will be slightly less than the supply voltage.
rx is an input , it will have a high impedance to ground, possibly with a weak pull up, so, a voltage measurement will probably show a value between tx and ground.