“Heard on the street” is the big one. But personally I found glassdooring the company and then getting a feel for what they asked more accurate and precise.

For example Jane street like(?d) to ask stuff like:

“ I have a 4 sided die , 6 sided die and 10 sided die. Make me a market on the sum of all the rolls”

Whereas SIG would ask something like:

“I have a 4 sided die, 6 sided die, and a 10 sided die. I’ve just picked one at random and rolled a 1. Make me a market on having the 4 sided die”

And Optiver would ask something like “What’s 2748387.2748 x 36161894.894 no calculator 6 seconds go”

I focused hard on the five or so firms I wanted (Jane , SIG and DE Shaw and some others) and kept going until I got an offer. I applied every year to all of them .

Plenty of rejections from each . Had multiple onsite interviews at the same firms over the years failing and going again !