I agree with Rich on the Accuvin and is what I use. You can also dilute the wine down with distilled water so that it makes it easier to see the color difference.
I've had some people tell me that using a bug light, those yellow bulbs, can make color changes in red wine more visible but I've yet to try it personally at this point.
- Jim
I hereby grant Waldo the title of 'Wine Whisperer.'
Waldo has a great point here - taste enough wines as you're making them and you'll get a feel for what a wine tastes like at the start compared to how it ends. If you're making the wine for your own consumption, this makes a tremendous amount of sense.