How do you know that medical devices are reliable, accurate, and safe?

Hi,
How do you know that medical devices are reliable, accurate, and safe?
I would like to evaluate several medical devices (for example: therapeutic ultrasound unit, interferential unit, cardiac defibrillator, laser therapy unit, etc.). The evaluation would include the reliability, accuracy, and safety of the medical devices. This would be important contributing to best practice procedures.
For example, I looked at AED (Automated Electronic Defibrillator) info by checking the FDA site: U S Food and Drug Administration Home Page
There are numerous defibrillators that are not approved/registered by the FDA. And several approved/registered units, have been recalled by the FDA, due to serious problems. Therefore, I thought the FDA is a very credible site to obtain an independent evaluation of medical devices.
I believe that approval ratings that include: 'CE', 'UL', and 'GS' are only electrical safety ratings, and do not evaluate the reliability and validity (accuracy) of a medical device. I believe that ISO certification generally refers that a manufacturer "has introduced and applies a Quality Management System"; "demonstrated compliance of the Quality Management System" related to a particular area/code/regulation/policy. This information does not readily show reliability, accuracy, safety of a product. And, any claims by the manufacturer could be prone to internal bias. This is where independent evaluations by the FDA provides strong, credible evidence. And, I noticed that several medical devices state that they are FDA approved/registered. However, medical devices are regularly sold without FDA approval/ registration.
Please let me know if I am mistaken with any information, and provide further advice.
Why would a manufacturer of a medical device not obtain FDA approval/registration? I believe that FDA approval would be an exceptional selling feature.
Is it best to choose a medical advice on the basis of FDA approval and registration?
Or are there other regulatory bodies that provide a recognised standard of approval?
I hope that this information would be helpful to share with all health/medical practitioners.
Cheers, Phil