This is a post by Evan Winter in his presentation.

Discusses how to preview assembly constraint status. Incomplete assembly constraints can be difficult to identify when previewing the assembly constraint status.

In the pic shown, the part of the model that is partially constrained is clearly highlighted in red. You can enhance the visible status of incomplete assembly constraints within your model by following these steps:

1. Open the System Colors dialogue box by opening the File menu, selecting Options, and then selecting System Colors.
2. Open the Graphics dropdown menu.
3. Change the color selection for Secondary Previewed Geometry.

Goto slide #24 of his presentation here

Suggested reading: PTC content

Content Source: Evan Winter, PTC