Is there any way to tell a poster real identity from a forum post?

Not unless they admit to who they are.

Or you're a IT geek site admin ;)

My accountant had a bad (he says false) review on a consumer blog site. He thinks it may be a competitor making him look bad. He asked me to try to find out before it starts to harm his business.

Now in this case you are talking possible libel action.
The libel laws are extremely confusing when it comes to internet forums and more so when blogs are involved.
Best advice would be to ask a libel lawyer.