How to obtain the saturation point of a soil layer from the volumetric soil water content data?
I assume you mean sensors that measure volumetric soil water content. The answer will depend on the amount of precipitation and the permeability of the soil. Soils don't reach full saturation in some climates - especially coarse texture soils. I would say that if the soil water content reaches a maximum value that persists for a period of time after large rainfalls (giving a flat top) and ideally reaches the same maximum level during multiple wetting-drying cycles that it is reasonable to infer that is the saturation value.
Ideally you would collect undisturbed soil samples of known volume (e.g in by driving a cylinder into the soil (gently as possible) to determine the pore volume and saturated water content.
Also be aware that many soil moisture sensors give readings that are dependent on soil properties and are not linear up to 100% saturation so they may not give good values at high water contents. It is best to calibrate those types on your soils.