How do you tell how much fluid left in a bic lighter?

To check the fluid level in a Bic lighter, hold it upright and look through the transparent plastic window on the side. If the window shows a clear or light color, it indicates that the lighter is low on fuel. You can also gently shake the lighter; if you hear a sloshing sound, it still has fluid. However, if you don't hear anything, it's likely empty.