Because of this variation, visually inspecting your engine bay is the most reliable method. Look for a valve with a cap that might be colored blue (though this is not always the case). Be careful not to mistake it for other similar-looking components. A picture of your engine bay will help considerably if you need further identification.
If you're unsure, it's best to consult a repair manual specific to your 2000 Ford F-150 or take it to a qualified mechanic to avoid damaging the AC system.