What do you verify in System Testing ?

Answer

System testing involves testing the software code for following

• Testing the fully integrated applications  including external peripherals in order to check how components interact with one another and with the system as a whole. This is also called End to End scenario testing..
• Verify thorough testing of every input in the application to check for desired outputs.
• Testing of the user's experience with the application. . 

That is a very basic description of what is involved in system testing. You need to build  detailed test cases and test suites that test each aspect of the application as seen from the outside without looking at the actual source code.