these will fail at one point.
so apart from checking for anything weird before takeoff, how can we tell if an engine might fail?
how do certified repair centers test them?
how long do these last.

Resistance in the motor will change with age. Also, you can take a photo tachometer to see if the rpm meets the as new requirements.

the motors itself will last very long and you can not really check them. The two bearings inside the motor can fail and you cant really check that, just spin the props before you fly and listen if they sound strange or rattle etc.. You can never be sure, they can fail with 50 flights or with 400 flights, its a matter or quality control of the bearings as well. You can only change them soon enough but lets be honest, i guess you wont have 100 flights until you sell the phantom3 for the Phantom4 : p I am a just for fun users and i will not change the bearings because the Phantom4 will be out soon enough before i reach critical flight hours.But you have to listen to the motor for strange noises of course. They replaced my Phantom3 and the new one has one pretty stiff motor compared to the other two, so its hard to tell if this is normal or not for beginners when even new Phantoms show different stiffnesses, you need much more force to spin one motor on my phantom3

great stuff guys, thanks!

In my opinion, take it easy on the motors and keep any dirt FAR away from them as one little tiny piece of dirt will work its way in the bearing and wreck havoc as i was the test subject and even with that the motor still kept on going on my flamewheel F550 till the day i sold it on ebay. Don't worry and enjoy your phantom !

Brushless motors .. when looked after... last for a long long time... bearing will wear before the motor starts too