When to bi-wire?

No, you cannot measure with an ohmmeter. The way to measure this is to put an audio sinewave generator through a small resistor and drive the speaker directly. Then measure the AC voltage drop across the resistor with an Oscope or true RMS voltmeter and calculate the speaker impedance from that voltage.

Sonically I like bi-wire but some speakers sound better with single wires and jumpers. Need to compare.

Does the crossover makes a difference? Like 1st order Thiels don't allow bi-wire but higher order designs use bi-wire. B&W recommends it for "serious listening" in their owners manual.