1297 YC companies (from here: http://www.ycombinator.com/press/)
860 YC companies listed as either active or acquired (from here: http://www.ycombinator.com/companies/)
= 33% failure rate
That's a lot higher than I would have expected, to be honest.
(78% active, 12% acquired, on a base of 800 companies)
Techstars and YC appear to have pretty dramatically different philosophies on selection, which influence this.
The giveaway is usually a blog or social media accounts that haven't been updated in years.
At the top of the comment, you should see something like:
username 1 hour ago | undown [-]
^
click here