What are indications that a tank is getting low, other than the gauge, when you are underwater or do you take that last breath and that's it?
!
As mentioned in most cases you will have very little if any warning in terms of work of breathing.
However there should be a big safety margin between common sense and OOA. Even without "gas planning" for the dive you know that the SPG will show a steady pressure reduction. If that's not happening the light bulb should go on. Similarly it does not take too many dives at all before new divers know that they are good for :30, :40 or whatever for a given depth profile. Assuming you have at least a time piece and all things being reasonably equal you should know when to surface because there is no air fairy.
When in doubt, get out.
Pete