When you are trying to login to PLC and getting the warning below that says there has been some changes in the code: 

You can click on the details to see which part of the program has changed. 

If you want to see the changes in detail, you can compare your plc project with target. Right click on your PLC project and select 'Compare PLC with Target'. 

This will bring up a separated window . You will see what's currently in the PLC on the right section. The folders that have changed is indicated in bold. Extend the folders and double click on the red marked POUs to see the implementation changes.

You can now see what has been changed in the program. If you like, you can accept the changes from Target to your project by using 'Accept' options on the upper right section.