Xdiagnose is described as 'GUI tools for diagnosing, repairing, and failsafing X.org issues' and is an app in the OS & Utilities category. There are four alternatives to Xdiagnose for Linux. The best alternative is arandr, which is both free and Open Source. Other great apps like Xdiagnose are driconf (Free, Open Source), Glxinfo (Free, Open Source) and lxrandr (Free, Open Source).
DRIconf is a configuration applet for the Direct Rendering Infrastructure. It allows customizing performance and visual quality settings of OpenGL drivers on a per-driver, per-screen and/or per-application level.
LXRandR is the standard screen manager of LXDE. It manages screen resolution and external monitors. You can plug in another screen into LXDE or choose to use a big screen projector. Local screen and extenal screen can be used at the same time.