That said, teachers are among the best qualified people to observe and note differences between children -- and to authoritatively state that a child is not behaving in a typical fashion. For parents of first or only children, a teacher's observations may be the first meaningful sign of a problem. Such observations should not be taken lightly.
If a teacher does suggest that your child may benefit from an evaluation, it makes sense to find out why, and to ask the teacher to document any questionable behaviors. If possible, observe the classroom, observe your child's peers, and make your own judgement as to the teacher's concerns. Your child may be up and running during circle time -- but if he's one of six who seem "out of control," the problem may be classroom managment, not your child.
If, after digging more deeply into the teacher's concern, you agree that something seems "off," an evaluation should be the next step. Evaluations including a qualified child psychologist or developmental pediatrician, an occupational therapist, a speech therapist and a physical therapist should be paid for(in the US) by your local school district or a county service provider.
It's important to note that, while an evaluation CAN turn up autism, is is just as likely to turn up more easily address issues such as speech delays or hearing problems.