How can they tell the difference?
Track record would be one obvious differentiator. YC has been around longer than any of the other accelerators, and it's produced larger companies. There's just a lot more data and a lot more people who have gone through YC, which gives us an advantage. Paul Graham is probably the most prominent founder of Y Combinator, but he was also prominent before Y Combinator, and that's the type of person you would want.