The only fairly accurate way to measure is 1 watt 1 meter anechoic...
even then, many speakers are not adequately impedance compensated. As posted above... 1 watt @ 8 ohms is 2.83 volts, 1 watt @ 4 ohms is 2.00 volts.... a big difference in power for rating a speaker... Many speakers are called 8 ohm that stray down to even 3 or 4 ohms and those measurements can be quite flawed....
Plus when put in them in a room setting, you know deal with peaks in the room, so someone could easily rate at its highest peak rather than 1k or over average frequency response.
Not sure that there is anyway to win here.
These are just a few of the problems when quoting sensitivity...
If they are using off the shelf parts, like a seas, scan speak, audax, etc, you could take a look of individual drivers and go from there... If manufactures are truly trying to give you a flat response, the lowest sensitivity of any single driver should be very close to the sensitivity of the entire speaker, but even that is flawed... Tough call.
Tim