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Here is a brief roundup of links describing common behavior characteristics and early milestones which may indicate giftedness in infants, toddlers, preschoolers, young children. (This list may be especially useful before IQ test scores tend to stabilize, around 8 years old.)

1- Characteristics of intellectually advanced young people,
...https://www.davidsongifted.org/gifted-blog/characteristics-of-intellectually-advanced-young-people/

2- Parenting Gifted Preschoolers (Milestones - typical development compared with 30% advanced)
...https://www.davidsongifted.org/gifted-blog/parenting-gifted-preschoolers/

3- 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)).

4- Characteristics and Behaviors of the Gifted

5- Characteristics checklist for gifted children

6-Tips for Parents: Helping Parents Understand Their Profoundly Gifted Children,
...https://www.davidsongifted.org/gift...rstand-their-profoundly-gifted-children/

7- Profiles of the gifted and talented which lists 6 different types, categorized by personality/temperament and achievement

8- Bertie Kingore, Ph.D.: High Achieving, Gifted Learner, Creative Thinker? (hat tip to sanne)

9- 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).

10- Different from birth, behaviours of young gifted children, "Strengths or admired traits vs Possible Problems" (archived, Canada) This list compares/contrasts positive and negative views of different traits and characteristics typical of gifted children. Think: Synonyms - Antonyms.

11- thread about Early Milestones - what do they mean?

12- SENG video: The Misdiagnosis of Gifted Children

13- book: Misdiagnosis and Dual Diagnoses of Gifted Children and Adults

14- old post with link to article comparing gifted characteristics and ASD characteristics

15- post with checklist comparing gifted and ASD traits (hat tip to BananaGirl)

16- post with link to Gifted Resource Center of New England (GRCNE) article comparing gifted and ASD traits (hat tip to Nolepharm).

Note:
When a website or webpage is NOT FOUND or has been changed and no longer contains the described content, check the WayBack Machine (internet archive) for a backup copy.
- Link: https://archive.org/web/
- Example in this 2018 post, which describes use of the WayBack Machine.

Back to thoughts in reply to your original post...
When thinking about the types of stimulation which may be most beneficial for an 18-month-old, consider that reading with your child, talking about the book, pointing to the pictures on the pages, asking the child to locate items from the story and point to them, talk with your child about how the pictures show specific things the words describe, etc.

The patterns of daily interaction with your child set the stage for future development. Having conversations about things occurring in daily life will help a child develop vocabulary, and learn patterns of conversation, including asking questions, listening for answers, sharing observations, agreeing and even disagreeing agreeably.

You may want to begin making lists of your child's interests, books you read with your child, books your child may help you choose at the library (often based on the illustrations at this age). Eventually this may change into making lists of books your child has read on their own.