In engineering, we get over this problem by using a reference signal. When you process a signal, you will always need a reference for this purpose only.
As I see,many people have answered your question about learning Fourier series.But still I think concepts like these are better understood when you read standard books.So you may find this book informative :
Signals and Systems by Alan V. Oppenheim
I recommend elementary mathematics book on the subject. They deal with the basics and not with technicalities. There are many and you select one you are comfortable with.