1) Based on flow rate and nozzle exit velocity you can calculate the minimal power for acceleration of the air. This assumes 100% efficiency of the fan and nozzle. Actual power will be more because they are not 100% efficient.
2) Estimate the power output of the low pressure turbine, which roughly equals to the power input to the fan. This could be done by connecting the LP turbine to a dyno and operate the assemble as a turbo shaft engine, or roughly estimate it by fuel consumption rate, fuel energy density, and thermal efficiency.
3) Model the takeoff process as a constant-power process, calculate the accumulated kinetic energy of the airplane and divide it by time and number of engines, which is a even more ideal case than 1). This ignores the most inefficiencies and should give the lowest number.
Remember, power isn't a concept tied to electricity. So the power to drive the fan has noting to do whether a electric motor is driving it or the LP turbine is driving it. The numbers are the same (if you live in a region where mechanical power and electric power are measured in the same units).
Thinking about it again, I think this question got the logic wrong.
Before the design work of a turbo fan engine, one of the most basic spec is how much power do you want to drive it with (other questions include bypass ratio and thrust).
This is not something you estimate after you have an actual engine. It's something you have to know before the engine is designed (and simulated when it's designed, and tested after it's designed).
The quickest way to know the power required to drive GE90's fan is to ask GE engineers.