One well known and reliable test is called a “fiber burn” test, for this:
1. Select single strands of hair from different areas of the hair extension you want to test;
2. Take a single strand and use a lighter to keep a steady flame and;
3. Pay attention to the odor, the way the hair glows and the flame.
Be sure not to use matches as they will mask the odor due to the sulpher.
Human hair is a protein fiber that burns with a small flickering flame and will not continue to burn unless you hold the flame continuously to the strand. It burns briefly with an orange flame and then chars leaving a dark ash that turns to powder when crushed. The odor is of burning flesh or burning feathers.
Synthetic fibers are either nylon or polyester and each burns quite differently.
Nylon fibers burn briefly but rapidly and then melts. It has a sparkling flame that has a blue base and orange tip. The ash is like hard amber beads. The odor smells like boiling green vegetables or burning celery.
Polyester fibers burn and melt quite rapidly at the same time. It has an orange sputtery flame with black smoke and gives off a sweetish odor. The ash drips and is sticky to the touch before forming into hard black beads.
Remember to burn test each fiber separately as your piece may contain a blend of both human hair and synthetic.