I'm amazed I can't find anything that displays the current frame which is helpful to study test timing and adjusting and knowing where it went wrong. Ideally I would like to show seconds and minutes and the frame number.
Enable the Metadata stamping in the render output properties. It also works for OpenGL renders.
Here is a 720p output example of what it looks like with a bigger font and the default options enabled:
Here's how to find the option in Cycles Render (first image) and in Blender Render/"Internal" (second image):