In this work, we describe a method to measure and characterize the mechanical properties of nano-electromechanical systems (NEMS) based on laser interferometry. The resonant mechanical modes of doubly-clamped nanomechanical beam resonators were first characterized using their thermal fluctuations in air. Afterwards, the NEMS devices were electrothermally actuated, and their frequency responses were measured, also in air. The main novelty in this work is the simultaneous use of two electro-thermal actuators integrated on opposite ends of the NEMS, which could selectively actuate higher order modes based on the relative phase difference (of 0°, 45° or 90°) applied between the two actuators. All the measurements were carried out in a homodyne Michelson interferometer, allowing for ultrasensitive non-contact, noninvasive, remote, and non-destructive analysis.