How do I verify a guest's identity before sharing check-in details?

The honest starting point is that you can't truly verify identity over a chat; you can raise the bar. The best cheap version is matching whoever's messaging against the reservation on your calendar: the name, the dates, ideally the phone number the booking came from. If someone messaging you isn't tied to a current booking, that's your signal to slow down before sharing anything.

A concierge with your calendar connected does this matching automatically and simply won't hand out access details to a number it can't place against a live reservation. That covers the ordinary case well. Where it can't help is the genuinely determined, a guest who forwards the code to a friend, or a previous guest reusing a static code. The only real fix for that last one is a smart lock that issues a fresh, expiring PIN per stay, so an old code is just an old code.

Manually, before you send a door code to anyone you're unsure about, ask for the booking confirmation number, check the name matches, and glance at their Airbnb profile for verified ID and reviews from other hosts. Push a little harder when the signals are off: the message comes from a different number than the booking, someone other than the booker says they're arriving first, it's a last-minute booking with no reviews and no verified ID, or they're asking for codes days ahead of check-in. And keep everything in your booked channel, because a request to move the code to email or a random social account is itself the red flag.