When I was interviewing developers from a very large offshoring firm, they covertly had an account manager listening in to take down the questions for future candidates. Each interview, I'd ask a slightly different variation and it became obvious what was going on when a candidate gave a textbook answer to the question I'd asked the previous candidate, including references to tables I hadn't mentioned and that had no purpose in the problem I presented them.
Also had an incident where I interviewed one person but am pretty sure someone different showed up and was completely clueless about the most basic tasks. Within a week I think they figured out that I was onto them and made some excuse to quit about taking a contract closer to home.
Honestly, at this point many American companies have an office of their own offshore, and it's generally considered more prestigious and better condensated to work for those companies directly, so I assume there's close to zero talent left in the large outsourcers.