Sometimes I think we have it wrong. Instead of trying to determine if someone IS a spammer, why not try figure out if they're definitely NOT a spammer.

So start with a pessistic view they they are, and that they need to be shown a CAPTCHA. Then do some analysis to try figure out if they're legit, e.g. time spent on page, mouse/keyboard interaction, geo-location, referrer etc.

If they're all good, don't show them the CAPTCHA (perhaps just rely on honeypot inputs), otherwise show them a CAPCTHA as a next step after posting content (and apologise in case it's a false positive).