When I went in for my screening I went in blind (no intel on what to expect), I'll tell you upfront, be as completely honest with yourself and the questions as you can. Don't worry about if somebody will think less of you for a certain answer or if you have certain beliefs that say you don't need to worry about issues (I made that mistake and short changed myself). Also, the shrink I saw had a little game they did with the folks they were screening (while you're in the office talking to them about some of your experiences someone will walk down the hall and slam into the door/wall to see if it screws with you- backfired on the shrink he ended up in a chokehold). I'd also suggest having someone take you to the appointment. I was so jittery and on edge I couldn't drive for a few hours.
I was diagnosed with PTSD and at the time I was drinking a fifth a day. I told the evaluator I thought I drank to much and was trying to fix that on my own, they offered me any help I needed and it didn't effect my status