This is actually an older feature request that has been on my list for some time now, but with all the Constant vs. Variable confusion, I feel it's even more needed than before.
It has been stated several times that there isn't a strict rule when to use the Constant and when the Variable Mode. If you want to be sure, you'll have to try out both. Every time.
Now, I don't have the newest equipment and compressing takes about 40 minutes (in Variable Mode) from start to finish. Doing that twice plus taking time to find and judge several sequences in the two resulting movies (let alone the needed harddisk space) is not practical.
So here's my suggestion:
Scan through 20-30 minutes in the middle of the movie, extrapolate the needed bitrate for the compression and note a dozen key-sequences. Those key-sequences are determined by the originals bitrate or quality factor. Have 4 extra low, 4 normal and 4 extra high bitrate scenes.
Now compress these scenes (and the surrounding 10 seconds) once with Constant Mode and then with Variable Mode and generate two seperate test projects, complete with all the .IFOs and VOBs. If you want to get fancy, put out side-by-side screen captures as .BMP.
The user can now easily judge the representative scenes with her software player or picture viewer.
If all this finding-key-sequences proves to be too much trouble, consider having the user enter a few timecodes by hand. Not really comfortable, but nice enough for a first shot.
It may even be a good idea to seperate this test-tool from the actual compressing app. That avoids confusing the real One-Click-Users.