I found this super interesting checklist online and it lists 20 signs you can look out for that might suggest your child is gifted. http://www.popsugar.com/moms/Signs-Giftedness-27331796 Besides the usual suspects like large vocabulary, good grades and great math skills, I liked that it listed a "great curiosity about objects, situations, or events; asks provocative questions" as one of the signs of giftedness. In such troubling times, it's important that this trait is included.
The main clue is that everything happens before it's supposed to. Early visual tracking and high alertness (especially 3 weeks to 4 months); early speech and very rapid vocabulary acquisition; early use of puns and verbal humor; early problem solving with puzzles and toys; and early reading, counting, and simple arithmetic, particularly if untaught.
https://www.nagc.org/resources-publications/resources/my-child-gifted/common-characteristics-gifted-individuals this is commonly used by educators and parents.