The Life Technologies Diamond Pro-Q and Pierce Phosphoprotein staining kits will detect serine and threonine phosphoproteins. Has anyone used these kits to detect M6P on N-linked oligosaccharides?
Pro-Q Diamond phosphoprotein gel stain provides a convenient method for selectively staining phosphoproteins (not phosphoglycoproteins) in acrylamide gels.
If you want to found all phosphoprotein (include phosphoprotein, phosphoglycoprotein and phospholipoprotein) you can use radioacrive phosphate labeling.
Also you can use enzymatic identification of mannose 6-phosphate.