In every ASD Facebook group or page that I am part of, inevitably someone new pops up and asks "We are having our child assessed soon. I've no idea what to expect. What do I need to do to prepare?"
My answer: Write down everything. And I mean everything.
Every little thing that you remember about your child from the day they were born about the way they did things, how they slept, how they ate, developmental milestones, behaviours that weren't quite right, things you thought were a bit strange but you couldn't put your finger on what was going on. Wouldnt sleep unless wrapped tight as a bug? Write it down. Screams at hand dryers in bathrooms? Write it down. Changes the type of voice used sometimes? Write it down. Cant handle paint colours being mixed together? Write it down. Takes a long time to respond to questions? Write it down.
Spend weeks doing this. Every time a thought pops into your head, a vague memory from years ago perhaps, jot it down. Write a catalogue of your child's behaviours and put them into groups. If you are having an ASD assessment, match them against the DSM5 criteria. Otherwise just put them into logical groupings: development, communication, sensory, behavioural, social. Print it up and take it with you to your first appointment and watch the doctor fall over in amazement and gratitude. It makes their job so much easier when there is a clear list of the issues at hand, and they take you far more seriously than if you (like so many) sit down and in a moment of anxiety forget every important thing you ever knew. Sometimes the Parent Questionairre they give you doesn't contain the right questions to elicit all the pertinent facts, or the clinician doing the assessing doesn't probe deep enough. Hell, if you have to, take videos of your child so they can see for themselves what may never present in a clinical setting. If you have everything that you know about your child in black and white in your words it will be more powerful than you can imagine.