I heard that Tesla could effectively "run" experiments in his mind, not just visualize them. Another pointer is that some "schools of thought" divide the art of thought into ability to visualize and ability to analyze where the latter has to be built upon the former and only when the former is perfected. My personal belief is that being a genius is about the ability to keep in mind many things at a time without losing details.

I read this about Edison.

Also he read very fast, the librarians though he did not like the books that's why he was returning all of them every next week.

Also what usually geniuses can do is process information a lot. Read and write many papers. Eg Terrence Tao. His blog is always going strong, the polymath projects, plus regular teaching, plus his own (other) research.

Alexander Grothendieck, wrote books full of brand new ideas about very abstract math. Amazing insights. Ability to work with unfamiliar cutting-edge things while keeping focus, while communicating them to others.

The most brilliant guy I ever knew was a founder of the field of computational geometry. He has probably co-authored 500 papers by now. he was a paper engine and would hold court with 10 or 15 people and they would all figure out how to prove the theorems in a paper. His students said that when solving problems he just never seem to to go in the wrong direction ever at all! At age 29 - already a full professor - he got the NSF Wasserman award - top researcher - all fields of science. His dad was a farmer in Austria. He was the first computer scientist to ever get the Wasserman award. Herbert Edelsbruner.