Someone from Quora can confirm this, but based on what I have observed, I don’t think they are using any third party tool for A/B testing.
I use a tool called Ghostery (browser extension) for this purpose. If the website is using any of the popular third party platforms for web-analytics, tag-management, ad-serving, or A/B testing, Ghostery will be able to detect those. Of course, if you are doing this in-house, it won’t detect anything.
This is what Ghostery detected for Quora:
So, it does not appear that they are using any of the popular third party tools.