We had two paediatrician appointments - the second six months after the first. Diagnosis was only made after the second appointment (and two and a half hours of questions, plus reports from SALT/OT/nursery/sight and hearing tests).
DS was present for both appointments, and climbed on every stick of furniture, tried to leave, played with taps, threw toys etc. Talking was a bit of challenge! Having someone extra there to sit with DS is a good idea. I wasn't worried about my DS understanding, but basically one of us could talk/listen, and the other had to concentrate fully on him.
At the end of the first appointment (in which the paediatrician had talked to us and asked some questions, and also observed DS and tried to do some simple tests, eg with blocks), the paediatrician asked us what we thought. DH and I said that we thought it could be ASD, and the paediatrician said, 'yes, we are looking at something in that general area'. This was what we had to keep us going for the next six months - nothing more specific than that. The first time anyone said the word autism was when DS actually got the diagnosis.
I don't know what the policy is in your area, but be aware that a diagnosis may still be months away.