I think a 10x team is one where a 10x programmer is able to set the technical direction, so you don't have a -5x codebase grow up organically (which is what you get when you let a group of devs of whatever skill level go off and just do whatever they think is right without considering the larger picture. The devs that are able to thrive in that type of environment will themselves become Nx coders (where N > 1 but may be more or less than 10), because they learn the good habits that are really behind being able to deliver good code consistently and hopefully never learn the bad habits that make a 1x coder. The more TDD they do, the higher that number will be and the faster the process will go.

I'd suggest you won't just find this in the wild, so you're probably better off screening for a willingness to do unglamorous tasks and an absence of the worst bad habits.