"The gravitational force is so weak that, from one perspective, it is amazing that we have noticed it at all. It is only able to become a strong influence on our existence because it is always attractive and cumulative. All of the atoms in the Earth conspire gravitationally to pull us toward the Earth's center, giving us weight, while the electrical forces of the electrons and nuclei of these atoms have opposite electrical charges and cancel each other out, so we experience no "electrical weight" from the Earth." https://www.npl.washington.edu/AV/altvw149.html
Another thing is the force doesn't vary over material or surfaces that would interfere with electrostatics, such as rubber, grass, fabric, etc. It's constant regardless of any materials inbetween and regardless of the charge of those materials. If you make something levitate with electrostatics and can measure the charge, it's completely consistent with gravity.
If you take a charge from a van der graf machine, why dont you get heavier or lighter?
Experiment to measure equilibrium between charge and gravitational mass. Huge mass on left, small mass on right, object in between that theoretically should be in balance. Could also mix in like charges and unlike charges.
https://www.texasgateway.org/resource/13-coulombs-law
Slight negative charge would quickly push you away from earth if it was responsible for the downward force
If gravity is real and earth is flat, then we would feel different forces at different places on earth because more mass would be on one side. This would be measurable with various instrumentation, even simple stuff, because it would be so pronounced.
An experiment to show whether seeds are reacting to gravity or electrostatics would be to put a strong electrostatic field (even one that will lift the seed) above it, and see which way the seed tries to grow. As far as I've found the field can effect germination and growth rates, but will not trick the seed into growing the wrong direction.