How to get my stammer to show itself?

How to get my stammer to show itself? I'm going to assume that this is opposite to the regular kind of question on this sub. I'm finally going to try for professional help with my stammer but need to get assessed first seeing as it's not currently listed as something I have. But whenever I talk about the stammer to a professional or in any way that it could be noted (e.g. doctor/teacher) it magically disappears and I can talk fine, then 5 minutes later it's back with a vengeance. So my question is, when I go to see this assessor tomorrow how can I improve the odds that this stammer will appear and not just leave it to chance that the pressure of the occasion will bring it out? Thanks.