Based on my experience keeping helping to keep Ravenbait's forum spam-free, I can offer the following:
Spam bots are basically scripts run by a spammer which search out blogs and fora with the intention of maximising the exposure of a website to search engine web crawlers.
Most of the time, they don't post anything at all - when they do, it is usually to advertise generic pharmeceutical drugs, or websites supposedly offering porn (but instead are more likely to download viruses into your computer).
If a spambot posts, it is fairly obvious, but for those that just lie dormant, clogging up the membership lists, here are few pointers:
1) Wierd usernames, sometimes created by just randomly hammering at the keyboard.
2) Obvious usernames, usually with key words like "online", "casino", "poker" or variations on generic drug names.
3) If a forum profile lists location and interests, there may be clues in dodgy spelling or syntax.
4) For those with moderator/admin priviledges, try running an IP check, which may reveal the server from which the spammer has posted. That may give an indication that the country of origin is not what is advertised in the profile.
5)The sign-in email address is a huge clue, and there are no shortage of threads on the web with extensive lists of email domains routinely used by spammers. The Whois look-up can be very informative, as it reveals the registrar of the domain. Certain domain registrars are synonymous with spam, such as Estdomains in Russia.
Hope this helps.
While we're at it, can someone delete the user "onlinepokerr", if they haven't been given the order of the boot already?