Easiest way I know is to hook them up and put a meter on the leads.
That gets you within the ballpark, maybe plus or minus 20%. An ohmmeter is just reading resistance. Impedance combines resistance, capacitance & inductance, but it turns out that impedance is sort of close to the resistance. Thing is, it changes with frequency. But, yeah, an ohmmeter will get you kind of close to the impedance of a speaker.
Yep, completely true. If they are being consistent then impedance would be measured at 1000 hz. I don't know if that is true or not. In my experience, resistance is usually a bit lower than impedance.
As you say, it will get you in the ballpark. Usually that is good enough. Anything below 2 ohms or above 32 ohms could be questionable although there are amplifiers/transformers that can handle those loads too.
Most amps that can deal with 2 ohms will be for sound reinforcement or maybe for bass guitar. I can't think of the model but there is at least one vintage Fender tube amp that has a 32 ohm output on the transformer. Since their amps were copied so often there could be others.