iPhone 6S, 6S Plus, 7, 7 Plus, 8 & 8 Plus
MRI - Mobile Resource Inspector
Apple Pay Presence Test
Battery Condition - Shows Cycles and Battery Health
Bluetooth Presence Test
Camera - Tests all cameras on device for functionality
Cellular Baseband Presence Test
Sensors - All other relevant sensors built into the device: Accelerometer, Ambient Light Sensor, Barometer, Compass, Gyro, Home Button Presence, Thermal Tests include the Number of temperature warnings.
Services Checked: Backup Method and last time backup was completed
Software Version check including Frequent Crashing App history
Touch ID test on applicable devices which in this case are iPhone 6S - iPhone 8 Plus
Wifi Presence Test

iPhone X, Xs, Xs Max, 11 Series

we do the following additional tests under the MRI test:
Face ID Presence Test
Wireless Charging Presence Test
Then we do an individual Face ID test which tests the functionality of the Face ID component and tests the following:
Camera Sensor Test on Rear Telephoto
Camera Sensor Test on Front 
Camera Sensor Test rear Wide 
Face ID Sensor Check 
Face ID Presence Check
Audio Testing is done on all our devices which tests the following components:
Front Mic (Display)
Rear Mic (Enclosure)
Bottom Mic - Left (Enclosure)
Bottom Mic - Right (Speaker)
Receiver (Display)
Receiver - Stereo Speaker (Display)
Then after all these are completed and results are taken down the Cosmetic check is done to note any and all physical marks, scuffs, dents, scratches and any other cosmetic ‘damage’ a device may have.