In usr/src/uts/common/fs/nfs/, there are some historical naming conventions that can help you understand where you are in the code:

rfs_

You are in the NFSv2 or NFSv3 server code. It may also be generic code that will call down into the NFSv4 layer.

rfs4_

You are in the NFSv4 server code.

rfs41_

You are in the NFS4.1 and/or pNFS server code.

nfs_

You are in the NFSv2 or NFSv3 client code. It may also be generic code that will call down into the NFSv4 layer.

rfs4_

You are in the NFSv4 client code.

rfs41_

You are in the NFS4.1 and/or pNFS client code.

You may end up in code which doesn't follow these conventions, i.e., the spe code I will be adding, some of the mirror mount code, etc. But if you are look at a stack trace, you would be able to see this type of code being called by a function following the above patterns.