you can measure with capacitance-voltage in solution if you can get a relative big size graphene, please refer to my article "Semiconductor properties and redox responses at a-C:N thin film electrochemical electrode" at Diamond and Related Materials, Diamond and Related Materials

another way is to use Kelvin prober

thanks Aiping

I am reading your paper but I need a formula based on graphene parameters.

I found that fermi level is zero for intrinsic graphene and has a relation for doped one

which can be write in the following form:

Ef = (planck_constant/(2*pi))*sqrt(pi*abs(n))

in which n is dopant concentration.

Unfortunately I am not sure about that.

Reza, Fermi level is refer to the level of vacuum. If you use semiconductor-electrolyte way, then you get the value referring to the reference electrode. Just a bit remind.

Good luck!