In short, you don't. Not definitively anyway. But the ladders are inspected before and after the test, and are supposed to be inspected after each use and monthly (yes I know in a volunteer department this isn't exactly a reality, but still.) Just like with hose testing, or any other testing for that matter. Hose is tested to 300 psi for 3in or smaller, 200psi for 4in or larger. I have tested hose on Monday and it pass, then use it on Tuesday at a fire and it develop bumps/pinholes.
Here is an older standard for ladder testing for anyone who cares that much. I don't think it changed much in the latest update.
Duane
