Potential energy will be infinite if unknown.
Use a test charge to determine the potential.
Use a point charge or the cross product.
If the force used to cause acceleration is not constant, use the Right-Hand Rule or the dot product.
When moving a particle from one region to another, it is moved slowly to prevent a decrease in voltage, to change a neutron to a proton, to avoid adding kinetic energy, or to make the test charge feel two point charges.