What are the symptoms of this problem? Anything that can be seen/heard/sensed without taking the barrel off?

Stretched, seperating brass are probably the most common symptoms. - Dan

When there is significant setback, the notch in the left lug can form a ridge in the upper seat which acts kind of like a detent. This can be felt when opening the bolt after firing or even with the action cocked. It is often possible to confuse this with other "feedback" however.
Often lugs will set back progressively and the ridge will be shaved away so is lost as an indicator. If headspace is excessive, the barrel should be pulled and theaction checked IMO. Regards, Bill.

So it sounds as if excessive headspace will always occur?

Well, theoretically yes, but with sane loads it will probably take a few lifetimes for it to present itself, assuming everything was kosher in the first place. - Dan