You've received great info already. I'll just add a few thoughts that came to mind when I read your post.

1) If you are looking for lists of common behavioral traits among the gifted... which may serve as "tells" in young children before formal IQ testing, here are a few resources which may be of interest:

Roundup of common Behavior characteristics and early milestone which may indicate giftedness
- Characteristics of intellectually advanced young people
- Parenting Gifted Preschoolers
- NAGC's list borrowed from the book A Parent's Guide to Gifted Children
((The first item on the NAGC list of Common Characteristics of Gifted Individuals is: Unusual alertness, even in infancy)).
- Characteristics and Behaviors of the Gifted
- Characteristics checklist for gifted children
-Tips for Parents: Helping Parents Understand Their Profoundly Gifted Children
- Profiles of the gifted and talented which lists 6 different types, categorized by personality/temperament and achievement
- Bertie Kingore, Ph.D.: High Achieving, Gifted Learner, Creative Thinker? (hat tip to sanne)
- A common trait in gifted children, often listed amongst identifying characteristics, is alternately described as: "advanced moral reasoning", "well developed sense of justice", "moral sensitivity", "advanced ability to think about such abstract ideas as justice and fairness", "empathy", "compassion". Links to lists of gifted characteristics include several articles on the Davidson Database here and here, SENG (Silverman), SENG (Lovecky).
- ages at which gifted children may reach developmental milestones
- thread about Early Milestones - what do they mean?
- SENG video: The Misdiagnosis of Gifted Children
- book: Misdiagnosis and Dual Diagnoses of Gifted Children and Adults
- old post with link to article comparing gifted characteristics and ASD characteristics
- post with checklist comparing gifted and ASD traits (hat tip to BananaGirl)
- post with link to Gifted Resource Center of New England (GRCNE) article comparing gifted and ASD traits (hat tip to Nolepharm).

Note: The WayBack Machine(internet archive) is often useful when a website or webpage is NOT FOUND or has been changed and no longer contains the described content. https://archive.org/web/

2) A book which may be of interest: A Parent's Guide to Gifted Children, by James Webb et al.
BTW, Webb readily acknowledges gifted children are often described as "intense."

3) The Davidson Database has lots of articles to search.

4) Although giftedness tends to run in families, there is the nature-versus-nurture controversy as to whether it is in the DNA and/or may emanate from parental interaction, including the enriching of the childrens' environment (for example, providing books and conversation which may help stimulate the brain and develop vocabulary).