Besides the apgar score, what determines a premature baby if the due date was in question, or if the baby was just small (5.5 lb baby at 35.5 weeks)?

Not sure of ?: Ballard score is used to estimate a babies gestational age at birth (weeks gestation). Apgar not used for this; it is an initial assessment of babies health at time of birth sometimes used to know if particular interventions (resuscitation) are needed and how successful they are.

Many things: There are many things to determine newborn gestational to numerous to list here. Weight and physical features play a role along with how the infant adjusts to its new environment. Based on weight "an appropriate for gestational age full-term infant is heavier than 2500 grams (about 5.5 lbs.) and lighter than about 4000 grams (about 8.75 lbs.)." this baby was good size for 35.5 weeks even premie.

Neither: Apgar scores do not accurately reflect the gestational age of ababy. Size does not always reflect age either. There are other scoring profiles that help us determine the gestational age of the baby. Check with your hospital to see which scoring system they use.

Exam: A well trained pediatrician can tell the difference between a 35 weeker and a term baby or one even earlier based on physical findings. This comes with time and training but is the same as most adults can tell the difference between a 1 yr old and a 5 yr old. There are specific scoring sheets that allow the staff to come up with the same info, but I don't find it confusing at all.