Dave,
For me, an approximative definition of when one could think of oneself as an expert (in this kind of context at least) might be when one has a sufficiency of knowledge, experience and skills that that which one sees in one's mind's eye at the beginning of a process is identical to its result.
As a consequence, I don't think of myself as an expert model boat builder (or expert owt else for that matter).
As it impossible for someone else to share a mind's eye view, calling someone else an expert cannot be based on the above. I would think that the expert moniker would have to be awarded on the basis of evaluating the quality of the result. As this would always to some extent be subject to the subjective opinon of others, its unlikely that full agreement in every instance is achievable, but I dare say that a general sort of consensus could probably be found.
Mattias