how can i measure liquid level

in any given image i need to measure the level of liquid If the bottle color is lighter that liquid i have sucessfully performed computation that how much liquid is contained in the bottle/ glass But if liquid n bottle have same color like water in white/transparent bottle I am unable to understand that how can I proceed

One way of doing it that I can do the edge detection and then find the distance between two lines( bottom of bottle n the height of liquid) or place a rectangle covering the whole image and the liquid contained.

Please help in this regard