can you see how many rotor magnets there are? Can you connect an oscilloscope and work out the number of voltage pulses per rotation? An analog voltmeter could work at a pinch if you turn it very slowly. You could also try shorting two phases (motor disconnected from inverter) and seeing how many 'stiff' sections per rotation there are. I've never tried that but it could work.
Some inverters want to know pole pairs which is half the rotor magnets.
Some inverters want to know pole pairs which is half the rotor magnets.