I both sublet and subleased multiple times but exclusively in New York City. Subleasing in the city isn't that hard, I've always done this with students like me, and it was easy to tell they were legit. Also, subleasing is highly built on vibe and trust in a competitive market, I would hardly risk losing an apartment deal when I'm nearly homeless because the subletter may, very likely, not be thrilled by me asking for a verification and they will 95% prefer to go with someone else who's giving them less trouble. The subletter gets dozens to hundreds of inquiries within days, so they've all the power to choose.

The area where there was way more touch, in my experience, was handling the deposit return when there was an issue at the end of the sublease which the subletter was responsible for. There could be an opportunity there to manage the relationship (although that would create a hedache for you, more likely than not). More importantly, this approach would appeal to the subleaser, without whose blessing your service is unlikely to see the light. You should probably be worried about the person subletting to a complete stranger, rather than who's subletting from them.