How can one tell the difference between the early stages of alzheimer's and forgetfulness?

Hard to say. As people age, they can become forgetful, but alzheimer's patients have this to a greater extent, and they have other noticable troubles. For example, they can have issues with simple daily living, like doing personal finances, organizing simple tasks, and keeping appointments. The biggest difference is that these things are progressive, they ger worse. They can be unaware of these defecits.
See your doctor. Forgetfulness can be normal aging or a part of another diagnosis, including dementia. It is best to consult with your doctor for a full evaluation.