Setup a buss named MONO and route it to your main soundcard outputs (not to a Master buss).
In the inspector, switch the interleave button to mono and reduce the input gain by around 6dB to
counter the increase in volume when when you switch a stereo signal to mono.
For now mute this buss.
Insert a send on your master buss and route it to your new MONO buss, remembering to switch it to pre-fade.
Now whenever you want to check your mix in mono simply mute the Master buss and un-mute the MONO buss.
Hope this helps.