Sometimes I need to verify which rendering API (Direct3D, OpenGL, Vulkan) a Windows program uses. Direct3D also has multiple versions and the exact version may be required.
For example ReShade needs this information (additionally Direct3D 8 is supported with a module), currently I practically have the guess the rendering API based on release date, cross-platform support and some other factors.
21 Answer
MSI Afterburner (may require the bundled Rivatuner Statistics Server) can provide this information when activating the on-screen display (OSD).
You can use Afterburner, if you activate the OSD for the framerate/frametime, it will tell you the API the application is using (D3D9, D3D11, OGL, etc).