I have simply used elemental CHNS analysis. When you use sulphuric acid for GO preparation, some low sulphur content is usually common (and accepted) - further purification doesn't decrease its content - it is expected by some authors that sulphur is in small amount present in groups such -SO3H.

EDX element analysis or an elemental analyzer (EA) was used to analyze the elements in functionalized graphene oxide.

elemental CHNS analysis

I am agree with Richtera. Otherwise you may go for ICP or XPS analysis. If you are following the Modified Hummer's method for the synthesis of Graphene oxide, then there is a chance for the presence of metallic manganese. So it will be better to go for XPS and ICP for final conformation.

XPS analysis is the best.

I also suggest XPS, its easy and effective.

Thanks for the answers. I also thought XPS would be the right analysis.

I prefer XPS

X-ray Photoelectron Spectroscopy is by far the best tool for this kind of job in my opinion.