How would you test iOS/Android application?

There are mainly two ways to test an app:

1.  Use a simulator (normally accompanied with the IDE)

2.  Use your device by connecting through USB cable to the laptop / PC and enabling Developer Mode.

you can follow the instructions that apple provides here:

https://developer.apple.com/library/mac/documentation/IDEs/Conceptual/AppDistributionGuide/TestingYouriOSApp/TestingYouriOSApp.html

but there's a new service out called "Test Flight", and it addresses this issue directly. Feel free to find more info here - http://testflightapp.com/

Regarding the Android Development, and since google provide us now with a specific IDE for Android development, Android studio comes with built in tools to apply your automated test which is based on the best Java Unit Testing Tool JUnit.

For the IOS development, XCode, and while creating your project, there is small check box to include Unit testing in your project.

But before that, you have to build your own code with some standards (Software Design patterns) which allow you to test your code.

i recommend using Genymotion simulators  , as they have a set of ready configured  smart phones so you don't need to gather information about the device and create the simulator by your self . 

https://www.genymotion.com/#!/

The best way to test an app will be to test it on a real phone.But, that is not always possible so as far as android is concerned .. 

1. Guess what are major smart phones which the people are using around. (say , Galaxy S4)

2.Now, use a website like gsmarena to gather h/w s/w related info regarding this smartphone

3.Use avd and create the a virtual device version which exactly (or nearly) mimics the real h/w s/w of the smartphone

Genymotion was my favourite Simulator for testing Android Apps. But because of some restriction on Genymotion , you can't test a few apps. Then real device comes handy for those apps. 

There are two ways to test 

1. You can deploy apps to apple device and test them.

2. Second way is best way to test yours app using Xcode XCTest framework. You can write your own test cases.You can record UI test cases by simply following methods. Apple bring lot of new things in Xcode7 You can explore this link for further info.

https://developer.apple.com/library/prerelease/ios/documentation/DeveloperTools/Conceptual/WhatsNewXcode/Articles/xcode_7_0.html