do you mean "how to demonstrate that my protein is actually a glycoprotein" ?

ys correct.

Hydrolysis (TFA), evaporation, silylation - GC analysis

Access the sugar composition of the sample will not allow you to know if the protein is glycosylated. To do so you should demonstrate that there is a linkage between the protein and the glycan. This can be done by several methods depending the material you have at your lab. Among the diffferent methods is electrophoresis: for example make the comparison of the electrophoretic mobility before and after deglycosylation, use a Glycan detection kit, lectins, etc...).