The purpose of the X test is to test impulsivity. I personally failed that test (out of the 14 X's, I hit like one). Various letters flash on the screen at varying intervals of time. The goal is to only click the spacebar when an X appears, not any other letter.

They will most likely ask you questions pertaining to ADHD symptoms. Depending on how thorough the evaluation is, there may be some tests that involve short term memory (such as reciting strings of numbers back to the evaluator). I know that with minors, they often ask for observations of the child from parents and teachers (if possible).