There is not a bright line. The formal autism assessments and surveys (ADOS, SRS, etc) give statistical probability. But kids move around over time and if a kid appears to qualify for autism diagnosis in preschool they may not when older. Or they may qualify more clearly when older. Autism is a dimensional trait, not a yes-or-no condition one entirely has or doesn't. It is also a dynamic trait which can be shaped by experiences and other traits and simple development over time. It is a description of one dimension of a person at one point in time.

The core distinction between autism and average toddler behavior is signicantly delayed development in social and communication skills. This can look like intense interest in something else (such as reading or memorizing information), or it can look like a motor control issue that affects speech, or it can look like sensory overload that makes social interaction unpleasant, or it can look like a global development delay that includes social delay also. Autism is the trait and not the cause.

As with most DSM categories, "autism" describes rather than explains; it says "here is what we see," not "here is why we see it."

For parents and teachers, the important question is: what are the specific concerns? Is a kid not learning to play and make friends? Are tantrums keeping the family from leaving the house? Are there behaviors which are unsafe or keep a kid from participating in activities?
Are there delays in motor skills or language?
These are all specific things that parents and teachers can act on. "Autism" is too vague to do much with.

To qualify as autism the social delay would fall in about the 1% most delayed group of kids; but DON'T assume this means the delay will be obvious. Many parents (us included) needed a teacher to point it out.

Tantrums are not autism; but they are common for kids who are frustrated by lack of social understanding, inabiity to communicate, and/or who find common situations unpleaant due to sensory differences. Autistic toddler tantrums are not different from "regular" toddler tantrums but are often more frequent and more lengthy perhaps because a child spends a lot more time frustrated, and has fewer other ways to communicate needs. But not all autistic kids have tons of tantrums or even misbehave much. Some kids have big social delays but are very compliant and not disruptive (relative to age).

Assessments such as MCHAT, ADOS, SRS are the way to see where a child falls vs "average" kids.

If worried about a toddler, the MCHAT screener is free online, and there's also a "video glossary" online showing what common autism signs look like.

The first sign is lots of crying and tantrum. They cry if you offer to play, offer to go to the playground, to eat, to swing, to walk to the next corner and lay down on the floor a lot. They cry even when lots of positive things offered.

My son was diagnosed as having borderline/mild ASD. Sometimes when he's tantruming over something insane, I wonder how much is ASD and how much is toddler-hood. Is it just a matter of degree and frequency?

I think a lot of parents with autistic children can identify an ASD child from just observing for a few minutes because we have seen it so often, but with some high functioning children it can be tough. The best answer to this question is to have the child evaluated by a professional. Some signs are bad eye contact, social impairments, stemming, lack of concentration, learning disabilities, aggression, and OCD like traits just to name a few. This is why ASD is called a spectrum disorder because all children are different and show different symptoms.