Current as of January 2017.

A phone's operating system can be found in 'Settings'.

It's a good idea to check whether the OS can be upgraded to a later version than the one that comes pre-loaded on your phone.

Apple iPhone


iOS = iPhone Operating System

Current iPhone models with operating systems

iPhone 6S (iOS 9.0)
iPhone 6S Plus 9 (iOS 9.0)
iPhone SE (iOS 9.3)
iPhone 7 (iOS 10.0)
iPhone 7 Plus (iOS 10.0)

Discontinued iPhone models

iPhone 5 (iOS 6.0)
iPhone 5C (iOS 7.0)
iPhone 5S (iOS 7.0)
iPhone 6 (iOS 8.0)
iPhone 6 Plus (iOS 8.0)
and below

List of iPhone models on Wikipedia - https://en.wikipedia.org/wiki/List_of_iOS_devices#iPhone

List of iOS versions on Wikipedia - https://en.wikipedia.org/wiki/IOS_version_history

Android


Android OS is used by many phone manufacturers.

Still Supported:

Lollipop (5.0 - 5.1.1)
Marshmallow (6.0 - 6.0.1)
Nougat (7.0 - 7.1.1)

No Longer Supported:

Eclair (2.0 - 2.1)
Froyo (2.2 - 2.2.3)
Gingerbread (2.3 - 2.3.7)
Honeycomb (3.0 - 3.2.6)
Ice Cream Sandwich (4.0 - 4.0.4)
Jelly Bean (4.1 - 4.3.1)
KitKat (4.4 - 4.4.4)

List of Android versions on Wikipedia - https://en.wikipedia.org/wiki/Android_version_history

Microsoft Windows


Windows Phone OS is used by some phone manufacturers.

Windows Mobile 5.0
Windows Mobile 6.x
Windows Phone 7
Windows Phone 7.5
Windows Phone 7.8
Windows Phone 8
Windows Phone 8.1
Windows 10 Mobile

List of Windows Phone OS on Wikipedia - https://en.wikipedia.org/wiki/Windows_Phone_version_history