Taking just 10 minutes and using just normal kitchen equipment, measuring the approximate concentration of levodopa turns out to be very easy. Meatyard [1] and MacDonald have identified the following process:

1. Put your test substance to soak in water. Levodopa, if present, will dissolve.
2. Put a mashed up banana to soak in water. Dopa oxidase will dissolve.
3. Mix the two solutions. If levodopa is present, the dopa oxidase will react with it to give dopachrome, which is orange/red.

The degree of orange/redness is the proxy measure of the concentration. The colour change is easy to see by eye. To measure it accurately would require a colorimeter. (If this measurement proves useful, I'll be happy to build an on-line colorimeter using a webcam - it probably wouldn't be very accurate, but better than you could do by eye.)

My guess is that this test is not 100% specific. There are likely to be false positives: solutions not containing levodopa which also turn red when mixed with dopa oxidase.

My own tesing of the idea is limited to:
- Sinemet which is shown to contain levodopa (though in the worse case the test could be being confused by the carbidopa which is also present);
- water which is shown not to contain levodopa;
- white vinegar ditto
- pea pods ditto

I would be grateful if people run this test on "interesting" substances, such as fava beans.

Reference

[1] Google dopa oxidase meatyard, which gets a document
BarryMeatyardDopaoxidaseforASE2.docx

John