The SDK can be accessed through Xcode 15 beta 2, and while developers do not have access to the Vision Pro headset itself as of yet, Apple will begin allowing testing starting next month.
Apple in July plans to open developer labs in select countries around the world, and some developers will be able to apply for development kits to test apps directly on Vision Pro.
While visionOS 1.0 can't be downloaded on headsets except by Apple employees, the software may give us hints of unannounced functionality the device is capable of.