How would I know whether the wheels are bearing too much weight? What does a wheel failure look like - is it like it bends and I need to replace it or are we talking catastrophic failures?The relevant factors are... Are you a large heavy person? Do you carry a lot of gear?
I didn't mean that you would know by looking at the wheels. What I meant was whether a combination of you and your gear are in the heavier or lighter range. I am not sure where the line should be drawn, but those wheels would almost certainly be fine for a rider that weighed 180 and was carrying 30 pounds of gear (not counting water and probably not counting food if you buy daily or at least as nearly so as is convenient). It would probably be fine for a bit heavier combination as well, but at some point it wouldn't be. I am not sure where the line would fall, but that should give you at least a vague notion.
So if you carry 70 pounds of stuff and weigh 250, I'd get some better wheels or at least a rear wheel. That would probably mean 36 spokes with a good strong rim.
As far as mode of failure, if a wheel fails it would typically be spokes breaking and would most often not be catastrophic without prior warning. More likely it would just mean stopping to replace spokes along the way, maybe multiple times on a long tour. If you don't notice and fix them you would ultimately wind up with a completely ruined wheel.