Yep, the vague question is to see how they react. Does he give you the irrelevant 20 questions treatment, ask you directly what the issue is, say he doesn't know (sometimes the most acceptable answer) or just blow it off? Does he realize offhand that your bike isn't likely to have sealed hub bearings? I wouldn't expect him to know about the bottom bracket at a glance (although RollCNY might), but he should be up front about what a un55 costs.