How to find servlet API version for glassfish server?

While coding a servlet I found a method that says

Since:
    Servlet 3.1

I guess that if I have the autohint from NetBeans to use it is because I have that Servlet version. But I cannot find a place to confirm that. I'm using glassfish4.1 as container. If I go to mypathtoglassfish4.1\glassfish\modules there I can see javax.servlet-api.jar and inside a manifest that says:

Implementation-Version: 3.1.0

Is that the proper way to check that? I'm especially interested in being able to tell my colleagues "go to that jar and check that property" so I'm sure that my code will run on their server.

As alternative, I found a webpage Oracle GlassFish Server 3.1 Application Development Guide that says: "GlassFish Server supports the Java Servlet Specification version 3.0." but obviously for Glassfish 3.1, and I couldn't find one of those for every glassfish version (not even for mine -4.1 )