Testing means measuring a devices parameters and comparing those to expected measurements. The goal is to find the real values and limits of the device and compare those to expected values.
We start rigorous testing by using these prototypes in the harshest manners possible. The main focus in these tests is to make sure, every functionality performs as expected. Secondly we test these systems' breaking points to have a thorough understanding of them. For example we test inhouse for higher ingress protection (IPxx) rating than it is expected, or have the system shocked at higher voltage than it is expected to endure. This way we know what the system's overhead is and we can make sure that these products don't just pass the third party tests, but they do it with with flying colors.
Thermal imaging cameras show us heat maps representing all components, or clusters that need careful examination, and possibly fixing, this way we can make sure that our devices will not have functionality problems after larger periods of time due to overheating. Signal noises are also a big problem for these systems so we have to make sure, that every possible factor inducing noises is removed, blocked, or cancelled for the final version of a device.