Like Lckrause said, it presents differently and there are different types. There are lots of different things that jumped out at me like attention span, determination, focus, passion for unusual subjects at what seemed like an early age, creativity, mechanical ability, early love of books...stuff like that.
But since I'm feeling a weird combination of bold and insecure right now and since you asked, some things that stood out to me were:
1. one of my kids spontaneously wrote legible capital letters at 24 months with no coaching, modeling or electronic toy help. he learned to write his first name at 27 months, after I dictated the last 3 letters a handful of times. Edit: Sorry, I happened to be rereading something in my journal and I got the age wrong. He wrote pseudo-letters at 24 months, spontaneously wrote legible capital letters at 27 months and wrote his name at 28 months. My mistake!
2. one of my kids did 100 piece puzzles right before he turned 3. the other one started trying to solve a 100 piece puzzle at 27 months, but could only do sections by himself.
3. one kid knew all the letter sounds (capital only) at around 19 months and would spend long periods of time searching adult books for his favorite letters (B and H) at that age. he learned lowercase when he got alphabet magnets for his 2nd birthday; it took him about a week.
4. one kid told me he found trapezoids in our ceiling at 27 months (only saying the word and excitedly pointing). He knew trapezoids from a Melissa and Doug shapes puzzle he had. He was always pointing out shapes inside other things.
5. my oldest drew very detailed pictures with perspective and multiple baselines at age 3. he went through a stage of drawing mechanical things (like our ceiling fan) as though they were taken apart. the drawings looked like something a 7 year old might draw.
6. both of them knew shapes, colors, and could (more or less) count to 20 before age 2. My little one still doesn't have the teen sequence down past 15. He skips a few numbers between 15 and 20. I'm not sure how they learned some of it. It's just something they picked up.
7. one of my kids could quickly add/subtract sums to 10 in his head at age 3.
Neither of my kids were early talkers or walkers and neither can read, btw. My oldest has always been very academically inclined. My youngest is a happy-go-lucky little guy who seems to fit in anywhere. My oldest was really into creating mechanical things from age 2 on. My youngest is not passionate about the mechanical stuff right now, but he seems very imaginative. He has an invisible baby dragon right now. So many variables...
But I feel pretty confident that they are some degree of gifted. I don't care either way, but I feel comfortable guessing that they are.
I'm not bragging. You asked and since I'm feeling, as I say, a bit bold and a bit insecure at the same time right now, I thought I would honestly answer your question.