Source: michaeljfox.org

What else could it be?  Early in the disease process it may be difficult to know whether symptoms indicate Parkinson's disease or a syndrome that mimics it.  Some conditions that could potentially be mistaken for Parkinson's include:

Medication-induced parkinsonism

Medications frequently associated with the development of parkinsonism are those often prescribed as anti-psychotics or for major depression.  Sometimes anti-nausea medications can also cause parkinsonism.  Unlike Parkinson's, symptoms typically occur on both sides of the body simultaneously.  These symptoms usually resolve within weeks or months of discontinuing the medication.

Essential tremor

Essential tremor is considered the most common neurologic movement disorder.  It is a chronic condition characterized by involuntary, rhythmic tremor of a body part, most typically the hands and arms, especially when they are being used for activities such as writing or eating.

Progressive Supranuclear Palsy (PSP)

Early onset of imbalance, frequent falls, rigidity of the trunk, and (eventually) eye-movement problems characterize PSP.  Symptoms usually begin after age 50 and progress more rapidly than with Parkinson disease.

Normal Pressure Hydrocephalus (NPH)

NPH is distinguished by a trio of symptoms: gait problems, urinary incontinence and dementia.  Experienced movement-disorder neurologists can generally distinguish NPH from PD quite easily.  Occasionally, a brain scan may be useful.

Also, WebMD.com offers a few other potential causes for abnormal nerve function linked to Parkinson's disease.