Uh, with a quick glance, just like the ASI?
Though at this point, if I want to unload to 0G, or pull up at ~3gs instead of ~5gs, I can more or less "feel it". But without the G meter, there wouldn't be any reference to get your feelings calibrated to in the first place.
Sims like CFS, Il2, they don't give you any indicator except black-out and red-out.
how do you use it in the middle of a fight?