Check ride
As a check airman, I am required to use 135.293 (A) (1 - 8) as guidance. Which means that anything covered by those itmes is fair game. Which basicly means anything. Realisticly it means the items that you use as a 135 pilot. Includes Part 61, things like medical, commercial pilot and ATP authorizations. Part 91, Part 135, systems, Opspecs, company manual and so on.
You do need to be familiar with it all. However, you are not expected to know all of it from memory. You are expected to know the things you use every day, like weather minimums, alternate minimums, systems, emergency procedures and the like. The things you don't deal with everyday, you are expected to know where to find them. A simple answer like "I do not know, but it is or should be covered by ( where ever it is)" and then look it up in the appropriate book.
Be short and direct with your answers. Do not give them BS or an information dump. That will just compound your problems. Just answer their question and shut up. Treat each question as itself. In other words, don't let the previous question and your answer effect your performance for the next one. Don't beat yourself up over one 'blown' answer.
By the way, each FSDO has its own list of hot items they hit heavy on. It depends on their experience with their operators, guidance from Washington and OKC and their own personal hot items.
Good luck.