if you have a speaker management device (like mackie big knob or presonus monitor 2 etc - https://ehomerecordingstudio.com/monitor-management-systems/ - i have the presonus monitor 2 https://www.presonus.com/products/Monitor-Station-V2 ) you hookup your single speaker (i use a powered avantone - http://www.avantonepro.com/Avantone-Mono-BLOK-Active-MixCube-Single-Self-Powered-Mix-Cube-Monitor.html ) and calibrate the level to match my other monitors (but of course turn it down via the management device when mixing... :-)). then switching between my monitors is easy, and pressing the mono button puts it all into mono. even then i switch to my stereo monitors in mono as well to make sure... then switch to stereo on the main monitors later to check bass and high end levels. another check is leaving it in stereo and switching back to the avantone to see of stereo signal is different than mono...

Check the mix through your cell phone. Ok, that's not really as cool.

So are you suggesting mixing in mono and panning each track until it sounds right. Then turning stereo back onto widen the mix using the side-channels?
 

Double-post

Panning in mono won't achieve much. Mixing in mono means exactly that. Pan everything centre and mix so you get the balance nice. Then put stereo back on and start panning things around. Some parts usually remain centred e.g. vocals, bass, kick drum etc but other parts can be moved.
 
I dont tend to work this way though. I prefer to mix in stereo and get the whole stereo picture set first then check in mono after that. Often some things need to be slightly adjusted for level. When I say check in mono I mean get a L+R summed mix into your mono speaker which I prefer to be a third separate device.

Thanks for the clarification.