Some commonly used testing methods:

Using the past experience in checking the design and performance.

Prototype testing.  Here  the  one  product  tested  may  not  be  representative  of  the population of products.

Tests simulated under approximately actual conditions to know the performance flaws

On safety.

Routine quality assurance tests on production runs.

The above testing procedures are not always carried out properly. Hence we cannot trust the testing procedures uncritically. Some tests are also destructive and obviously it is impossible to do destructive testing and improve safety.

In such cases, a simulation that traces hypothetical risky outcomes could be applied.

Scenario Analysis (Event -> Consequences)

Failure Modes & Effects Analysis     (Failure modes of each component)

Fault Tree Analysis (System Failure -> Possible Causes at component level)

What if there is a combination of factors?

All Analysis pre-suppose a thorough understanding of the physical system