By having a much higher launch cadence and then analyzing the flight hardware afterwards.

Also, they don't have anything human rated going beyond LEO. Coming back from the moon means you're going significantly faster and thus need a better heat shield

... so the real problem is that to get back and slow down nicely would require so much more launch mass, right?

(By slow down I mean to change to an orbit that has more drag and wouldn't take forever to return to Earth.)