A light beam propagating through the atmosphere has its amplitude, phase, and propagation direction changed along the optical path due to atmospheric turbulence. This effect is caused by the randomness of the air refractive index and depends on the local temperature, pressure, and humidity. The physical parameter that quantifies the turbulence strength is well known in the literature as the refractive index structure constant (Cn2). In this paper, a simple and low-cost technique based on a triangulation-like method to measure the Cn2 is presented for the first time to the best knowledge of the authors. Through a novel device comprised of three photodetectors in a triangle-shaped array placed on the receiver plane, it is possible, after power measurements and applying a developed mathematical model, to obtain precisely the position of the beam spot. Therefore, calculates the Cn2 and evaluates the atmospheric turbulence strength. Furthermore, the technique is useful to check the suitability of a free-space optical (FSO) link in a specific geographic region by monitoring the local turbulence. Also, the device will be useful to improve the deployment and maintenance of FSO systems.