The color of Graphene oxide dispersion in water is dark brown or yellow (when diluted). You can also check UV-Vis spectra. GO gives a broad absorbance peak at 230 nm wavelength. In addition, AFM, XRD, XPS, Raman spectra etc. dives distinct character of the GO compared to graphite or graphene. 

X-ray diffraction is the easiest and I think the better way,to ensure that you have GO. You must observe a diffraction peak around 10 degree in 2 theta. Raman is not always available but if can do it, it provides distinct  D and G bands, the D band appereance will tell you about GO. XPS tells you about O/C ratio, and high resolution XPS could inform about C-C double bonding and C-O in differente functional groups. FTIR is also a very good tool, it would provide the presence of the absorption bands related with the functional groups. You could refer to the attached article.

Article Graphene oxide powders with different oxidation degree, prep...

Before asking this question, have you done sufficient literature survey????

That is a good point, Subrata Gosh and I had stressed it before about other questions. Even RG is a nice meeting point intended to find answers, look for references, advice about confuse data/results, discuss about experimental issues people is experiencing and make research nets, it does not relief people -mainly students- from literature screening.