When reviewing poconopenn's high and low D photos, other than the location of the 'D', there are two more details that I noticed.
1. The thickness of the denomination - The '500' is thicker on the high D.
2. The low D type has extra white fur on the panda's neck.
Here they are side by side.
For me, it is very easy to determine which is which in first look by looking for the extra white fur.