how do i know which version of Windows I have on my Mac Book Pro 15" early 2011

The Mac comes from the factory with MacOS pre-installed. The particular Mac you cited would have shipped with custom 10.6.6 or possibly 10.7

Windows does not come pre-installed on any Mac. You have only the version you paid for and manually installed.

If there is some controversy about which version is installed, fire it up and ask it what version it is.

You should do what Grant Bennet-Alder said.

You could also try:

https://support.microsoft.com/en-us/help/13443/windows-which-operating-system