Dave Kurlan's OMG Assessment http://www.objectivemanagement.com is the best I've seen.

It's not cheap but it is accurate. And I have taken it myself.

The problem with using personality tests or, more accurately termed, behavioral indicators as a tool for assessing salespeople, is that they are not constructed to discover sales ability.

You can, for instance, get a behavioral indicator to tell you that a CFO scores high in "Drive." But what does this really tell you? Put that CFO in a sales position and maybe they'll fold up like a wet noodle--they may well be "driven" to excel as an accountant and decision maker, but not in a selling situation.

As for relying only on conversations and your gut feelings...the best talkers will win that one. We all think we're great students of human nature, but the reality and the facts are that we'd have better luck pulling the name of our new hire out of a hat.