We all know pretty much what we all have or could have. There is a process of evaluating what is possible with known materials and processes that will predict where "breakthroughs" will happen. We can look at a Chinese 5th gen fighter and know what they know about aerodynamics, control systems, and stealth. For example, if a Chinese aircraft is basically aerodynamically unstable based on its shape, we know they have advanced control systems. That doesn't mean there aren't surprises, but usually not huge ones.