Visual Studio has a fantastic internal diff tool: https://www.visualstudio.com/en-us/docs/tfvc/compare-files
I'm using Perforce in Visual Studio via the P4VS plugin. The plugin adds the "Diff" sub-menu when you right-click on a file's tab. But by default any diff operations are carried out by P4M, rather than using Visual Studio's internal diff tool:
To it's credit, P4VS does allow for the selection of a different diff app. But the provision is for a command line operation. I don't know how to make it use the current instance of Visual Studio's diff tool. Is there a way to do that?