In the code, you can check to make sure that vtkRenderer::New() is giving you vtkRendererOpenGL2:
std::cout << renderer->GetClassName() << std::endl;
For rendering benchmarks, you can also check driver information. Especially if you are benchmarking in different environments or between different builds of VTK. This must be called after you have called Render(), not before:
std::cout << renderWindow->ReportCapabilities() << std::endl;
The 60 fps that you are getting is just the frame rate of your monitor. The GPU is capable of rendering much faster than that, but the OpenGL driver is synchronizing to the monitor.