The process of occlusion starts at the surface and works its way upward in the atmosphere
Aloft, the occlusion process is denoted by the formation of a closed low, temporarly first at 700 mb, then at 500 mb etc. Thus the transition of a 500 mb short wave from an open trough to a closed low is an indication that the surface low is either an early occlusion or about to become one.
On thickness charts, the occlusion is coincident with a thickness tongue on the cold air side of the intersection of cold and warm fronts. The intersection of the occlusion with the cold and warm fronts is called the "triple point."
A classic occlusion has no temperature contrast on either side of the front, and is entirely in cold air.
Surface Pressures and 1000-500 mb Thicknesses (Analyzed)