What kind of edge differences are you looking for? What do you mean by internal edges, the shared edges of the sewn (joined) surfaces? Are you wanting to have tangent/curvature edges appear in a different line font than hard, non-continuous edges or are you wanting analysis types of differences?
If you're essentially wanting to check for gaps (free edges), then Examine Geometry is what you want(Analysis->Examine Geometry). If surface quality is a must, then just click on the Set All Checks button at the bottom & you will be given a text output of the results.
If surface quality isn't really much of an issue, then just check the Sheet Boundaries box & then OK & NX should highlight all free edges with a colored outline.
If you wish to display free edges on a sort of permanent basis, about the only thing you can do is change the line color, font or width for smooth edges. However, do this will NOT assist in determining free edges, it basically shows tangency & above when met. If that is more along the lines of what you want, click Preferences->Visualization then click the Visual tab. Check the Smooth Edges box then just to the right & down a little are the color, font & width settings.