Usb camera lib that will allow to measure camera latency?

I would like to be able to know how much my eye tracking USB camera latency is. Which is before it is decoded and displayed on the LCD. Both take time, LCD alone may take 25 or more ms to show the frame.

A better method would be to have a Python camera lib that can tell when a new frame in a stream is ready and I could log that instead.

Is there anything suitable, with Python bindings?

Not quite the answer to your question but a camera with very very low latency is the Sony playstation eye camera for the playstation 3. very good and cheaply available.
DirectShow driver for windows from internet archive here:

Howto here:

The directshow driver means it can be used by Skype etc.

An alternative is the ipisoft driver - they do live mocap from cameras and use this camera for same low latency reason. Info on this here:

This driver does not use DirectShow and so is not so easily discoverable by desktop apps.