Play a perfect fifth on a piano. Learn to recognize the characteristic sound of the fifth by paying attention to the differences between a fifth and other intervals. Compare the fifth to a fourth and an octave, as these are two intervals that commonly cause confusion.
Match the pitch of your instrument with the pitches of the piano. With string instruments, you may alternate between two adjacent strings or play them simultaneously as a double stop to check pitch. Using a piano will ensure you are tuning to the correct pitches.
Continue to practice tuning your instrument with the piano as a reference until you have memorized the sound of a fifth. Once you have memorized a fifth, you will no longer need to use the piano for reference.