I am a moderator of another forum.
On that forum I view the details of all new members, this will often give a clue as to whether they are real or spammers.
Links in the profile to advertising websites are a bit of a giveaway, as are certain geographical locations.
Usernames like "phonesforyou" or super discount anything are an obvious giveaway.
I suspect that a lot of spam is from automatic programs, though some is human generated.
A post containing no new information or enquiry is a bit of a giveaway, some spammers build up a post count by qouting say 10 existing posts and simply saying "great information, thanks for sharing" They later edit such posts to contain an advertising link.