So, if you read an article saying "Hire an SEO consultant today to improve your site visibility x100" and the author happens to be an SEO consultant, it would be an ad hominem to point this fact out ?
not ad hominem: his argument is invalid because [reason]. he ignores [reason] because he is a SEO consultant.
But suppose I don't know anything about SEO. How would I judge if the article is bullshit or not ?
If, however, I see "x is an SEO consultant" then I would at least suspect that the author is trying to sell me something rather than inform me.