You can't tell for sure by looking, not with enough certainty to ignore the tags that the artist set themselves. That rule isn't going to help you.
I was pretty sure that we tag "what we see" not "what the artist tags" here. I mean, there are artists that tag "loli" for works that do not follow Danbooru's definition of it.
But I guess you would know better about those rules since ye be a builder. So I guess this is just a misunderstanding on my part.