According to the obj file specification it should be a string after the 'g' command, but from my experiences each obj exporter handlehandles those things specifically.
It might help to add link to obj file in your first post.
EDIT
According to the model file, those wheels are groups with numbers 13, 14, 24, 25 (proofProof: image)
So those faces (and according vertexes, normals and textures) after line g part 013 to the next line with g command is the mesh of the wheel and so on..
I'm not using JMonkey so I don't precisely know how to rotate those parts, but you could cut the car mesh into two files (wheels and rest) and then iit should be easy to animate..
