You can demonstrate that by making Hall measurement before and after transformation.  Also in 3 terminal device configuration it can be understood by measuring resistance as a function of  gate voltages. 

Although Hall is good technique to find conductivity types, it becomes confusive for 2D materials specially in monolayer. I would like to go for resistance vs gate voltage technique. But in three terminal device, how resistance varies with n type or p type graphene?

Graphene shows Ambipolar R-Vg characteristics . By observing the charge neutrality point ( maximum resistance )you can understand the conductivity type of your graphene device. 

Vd = 0 represents undoped graphene. 

Vd = +ve represents  p doped. 

Vd = -ve represents n doped. 

Vd represents the gate voltage at the charge neutrality point.