Method and system for identification of a changed state of a fluid with respect to a reference state of the same fluid, the fluid having an optical parameter changing with the change of the state of the fluid. The method comprises: a) providing an optical arrangement including a transparent enclosure with a portion of the fluid, and an object observable through the optical arrangement, the arrangement being designed such that an image of the object in the changed state of the fluid is optically distinctive from an image of the object in the reference state of the fluid due to change of the optical parameter, at least one of the images being predetermined; b) illuminating the object with diffuse light; c) observing a current image of the object though the optical arrangement along an optical axis; and d) comparing the current image to the predetermined image to identify the changed state of the fluid. The comparison and the identification may be performed by eye or by a sensor with a logical circuit.