In a method for determining charge and/or size of an object (15) suspended in a fluid, the object (15) is introduced, together with the fluid, into an electrostatic trap (1) defining an electrostatic confining potential. The thermal motion of the object (15) in the fluid is observed under influence of the confining potential, and charge and/or size are determined from the observed thermal motion. In particular, the viscous drag on the object yields a measure of its size, while the stiffness of its confinement can be compared with a potential model to reveal the total charge it carries. Also disclosed are an apparatus and software for carrying out the method.