Psych Eval today!
wow....you'll surely be exhausted after that! Mine was for an hour and basically we discussed why I chose to go this route and how I am aware of the ways in which my life will change and how I have to change my relationship with food. It wasn't difficult or upsetting, so don't worry about it....you've already done your thinking and I suspect that you already know how things will be and have to be after surgery. The therapist is on your side, not against you, so go with the flow and let yourself learn more about who you are and what drives you.
Good luck.....you'll do fine!
Good friends are like stars.
You don't always see them but you always know they're there. 
Good luck! I had mine in April. The first hour or so was just answering questions and discussing the surgery. Be honest and realistic. Start thinking about healthy things you can do besides eat to handle stress, because that is a big thing they will ask you. They will ask you about your support system, family history, your own medical history and your expectations for the surgery.
The second part was the test-taking portion. I took the MMPI and some other smaller tests. The MMPI is a 567 question test. You will find it to be very bizarre. It asks strange questions using tricky wording. Just answer honestly about how you feel right now, not in the past. I know someone who failed because she was answering about how she has felt at different points in her life, and I guess it found her answers to be incongruous. The results will tell you if you have any personality disorders. The results also tell you if you aren't being honest, so be sincere.
I had to schedule a short appointment two weeks later to discuss the results. It took about 15 minutes (but of course my insurance was charged $150 for those 15 minutes!). He went over the results and told me I would be a good candidate. It was a weight off my shoulders.
Best of luck! I'm not sure if your experience will be the same as mine, but I'm guessing it will be similar.
Every psychologist and counselor does it differently. I'll be putting together my own battery of ASMBS approved tests for example, based on my research and networking. Each of us comes up with our own. Typically, it will always involve an assessment regarding your relationship to food. This may be conversational, or it may be quantitative. They may administer a test called the MMPI, which is used to determine potential degrees of pathology based on your personality. Many of us didn't have to take the MMPI, however. I'd argue for most people it's overkill by a wide margin. You will be screened for depression and anxiety. You will also be screened for anything that would deter from post-operative compliance, such as psychosis, or disordered eating.
Being "screened" may take the form of tests (MMPI, or the beck depression inventory, for example), or a questionnaire. It's less nefarious than it sounds.
The best way to prepare is to come comfortable and relaxed, and honest. It's really not so bad. Of course, being a counselor myself, I am nerdy and enjoyed my assessment, lol.
I follow a ketogenic diet post-op. I also have a diagnosis of binge eating disorder. Feel free to ask me about either!
It is not that we have so little time but that we lose so much...the life we receive is not short but we make it so; we are not ill provided but use what we have wastefully. -- Seneca, On the Shortness of Life
