Good advice if you can find them a small project. In other words: "date your future wife before you marry her".
In the situations I faced, we were looking for the co-founder/VP Business+Sales, so it's not a position where you say: "ok, try for a month and then we'll see". You sort of have to be sure because when you offer the position, it's a committment.
If you decide not to partner, pay them cash for their time.
If you decide to partner, pay them with equity.