The easiest way to find the highest and lowest notes in your range is to test them against a piano or keyboard. Even if you don’t intend to play any instrument, it is important to understand the layout of a keyboard in order to identify notes and understand the relationship between them.
A keyboard is tuned to concert pitch, which is an international standard for identifying notes. This enables musicians in bands and orchestras to play in tune with each other. When you sing, your perception of pitch may change depending on your state of mind or how tired you are. For this reason, it is important to have a reference from an instrument tuned to concert pitch.