How to check whether or not your GPU is currently using Mesa for rendering OpenGL/Vulkan?

As a Linux fan, I want to get into OpenGL development as a hobby of mine. I know that OpenGL is just an API that the GPU vendors must implement. Some GPU vendors' OpenGL/Vulkan implementations are proprietary, whilst some are open source (like Intel). Because I like open source, I want to make sure I don't use anything proprietary when I develop stuff, so how would I go about checking whether or not my GPU is currently using Mesa for rendering? The reason I am asking is because I've gotten mixed messages online, as I have heard that you can apparently still have Mesa installed but the GPU will be using something else that is proprietary, which is why I wanted to ask this question. Any help would be appreciated.