It is quite difficult to tell without adding further checks
I would normally add a question that a computer can't answer. Ie
"If today is Thursday, what is tomorrow?"
"What month comes before October?"
I use this technique on our forum as well as a Captcha and it has reduced the amount of spam quite considerably. Just having a Captcha wasn't enough and I was still seeing a lot of sign ups.
If your forum is being spammed, you need to also look at why it is such a nice target for spammers. Do you allow links posting for new users? Restrict this and other features that spammers love.