This thread may be of interest:

How to measure levodopa and dopamine content?

A simple way to detect most tampering would by a visual identification: look at the shape, colour and any markings on the pill .

Also, not conclusive, but certainly suggestive, you could accurately weigh a pill. I think it unlikely that a placebo will be close.

Alternatively, you may wish to consider natural reasons for what you are experiencing. You mention a "placebo". Does this mean that you're taking pills, but they are having no effect. Three things come to mind here:
- your dose is too low to cross the "on" threshold. What is your daily regimen, doses/times etc.? As the disease progresses doses that once worked may no longer do so.
- the drugs are interacting with protein in your diet.
- you are on a new generic which has less levodopa content or, perhaps, a lower bioavailability of levodopa.

John