Ask your instructor!
The short answer is use the legend on the chart.
The Class D become Class G airspace from whichever altitude the Class G normally goes upto.
If you look at the chart the majority of the Class D airfield have the shaded magenta lines around them indicating Class E begins at 700', so in that case Class E would begin at 700' (Class E normally begins at 14,500' MSL)
The Class E stubs however, I believe stay as Class E as
Extension to a surface area.
There are Class E airspace areas that serve as extensions to Class B, Class C, and Class D surface areas designated for an airport. Such airspace provides controlled airspace to contain standard instrument approach procedures without imposing a communications requirement on pilots operating under VFR.
Therefore, it is for separation and may be being used by another authority such as a centre for IFR traffic which may be going into the airfield, even if it is shut.