Perhaps the easiest method is to check the UV spectrum of the solution. A peak at around 270 nm is expected for rGO. There are many reports in the literature and you can refer to.
if the graphite powder is exfoliated properly, you will see a shiny layer on the top of your solution, very similar to the layer that you get when you have diesel or oil in water
an easy way is measuring pH. GO solution is an acidic solution due to oxygenated functional groups in rage of 2.5-4. graphite solution is neutral one in rage of 5-6.