[wz3chen]

Some indicators I've witnessed as a student: They started programming at an early age because they thought it was fun/interesting rather than thinking about a getting a job or making money. Have impressive side projects. Have competed and done well in algorithm competitions. Have full time software development experiences before starting university. High grades at an elite highschool. IQ above the rest. Educated and/or supportive parents or teachers.

[mildavw]

I'd add "found programming so interesting they did not finish high school/college." The maximum level of education achieved by my team ranges from middle school diploma to PhD.

[brador]

That's a great list! I'll add that they almost always have a story of how they hacked something when young. Maybe hardware, maybe software, but always to get something they were otherwise denied.