Here is a roundup of common 
Behavior characteristics and early milestones 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 - 
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).
WayBack Machine (internet archive) - 
- link: 
https://archive.org/web/- Useful when a website or webpage is NOT FOUND or has been changed and no longer contains the described content 
- Example in this 
 post.