There are a handful of ways to detect O-GlcNAc. This image describes a new method that reveals what proportion of any particular protein is modified, and of those that are modified, exactly how many GlcNAc residues there are per protein. They do this by adding a heavy tag (polyethylene glycol) to the modification, which makes the protein sluggish on a polyacrylamide gel. 

Rexach, J. E., Rogers, C. J., Yu, S-H., Tao, J., Sun, Y. E., and Hsieh-Wilson, L. C. Quantification of O-glycosylation stoichiometry and dynamics using resolvable mass tags. Nat. Chem. Biol. 2010, 6, 645–651