Assuming that the case in question is a bottlenecked rimless case, then the defining symptom of excessive headspace is a post-firing base-to-shoulder dimension that is significantly longer that that of the unfired brass.
Beyond measuring that particular dimension precisely, syptoms of excessive headspace include excessively flattened and spread primers (primers which appear to fill the radius of the primer pocket) and a bright stretch ring just forward of the head of the cartridge where the brass becomes thin enough to be pulled partially apart as the base is pushed backward against the bolt face.
"Excessive" headspace is the condition of the brass failing to be held in sufficiently close contact with the bolt face. "Excessive" headspace can be caused by brass that is too small or a chamber that is too large. "Too small" or "too large" are relative standards. A chamber which is overly-long compared to SAAMI standards has no excessive headspace if the brass used in it is tailored to its dimensions.
