Pass 1A along the trace, measure the voltage difference with a multimeter (it will be microvolts or

millivolts), compute the resistance.

Thin PCB traces will warm up with 1A flowing, increasing the resistance. Very fine trances might

overheat, try 100mA if unsure.

Having a current limited power supply makes this sort of thing easy (setting a constant current).

Alternative is use something like a 10 ohm power resistor in series with a low voltage supply to

produce a suitable current, but you'll have to explicitly measure the current (measure the voltage

across the 10 ohm resistor for this). In effect you have a voltage divider with 10 ohms and

the pcb trace.

Tables of pcb trace resistance for standard plating thicknesses are available online. Copper

always has a resistivity close to 1.7e-8 ohm-m, so you can calculate the approx resistance from

the dimensions easily enough.