Thank you for the answers so far. It seems that there is no way surefire way to tell from a comparison of the materials themselves. My sense thus far is that plastic tends to have a higher gloss than ebonite and the edges tend to remain a sharper, but this has failed me as a heuristic before.
What surprises me is how little variety there seems to be in the designs of feeds now. No one even seems to do "lengthwise" feed designs like on the Pelikan 400 or Montblanc 254. I am a big Montblanc fan, but not of the 146 or 149 for aesthetic reasons. The pens I love are the 1960s 1x/2x/3x models as well as the very early 1970s 22x--especially the first version of this pen. Both of these pens have very unusual feeds, even if you compare them to other "internal" feeds like the Lamy 27, Aurora 88 or Parker 61. They seem to me plenty juicy even on BB nibs.
The feeds on the 1x/2x/3x is especially clever, it seems to me, since they have some scope for flexing to maintain tension. (For those that do not know them, they are a bit like a banana with a double row of teeth running the length of the banana.) The "fins" or "teeth" of the nib can become brittle in my limited experience though.
I am also a bit surprised that more experimentation with the color of the feed is neither made, nor accepted. (Perhaps it is and I am unaware of it.) The Aurora Optima Auroloide I have with the new red-brown ebonite feed seems to have annoyed a lot of people, though I like it very much.