How I can simply measure interwinding capacitance in a transformer with electrostatic shield
One way that I can think of would be to connect the LCR meter to the shield and to one of the primary or secondary terminals with all other ports on primary and secondary open. 

One thing that may get in the way of a capacitance measurement like this is inductance, so try and minimize wire length to the transformer. This would give a rough idea of what the capacitance is.