I only use charcoal, myself, but...

...I thought this was a great tip

KITCHEN TIP: Checking Fuel Level in a Tank

There's nothing worse than running out of fuel halfway through grilling. If your grill doesn't have a gas gauge, use this technique to estimate how much gas is left in the tank. Start by bringing a cup or so of water to a boil in a small saucepan or glass measuring cup (if using a microwave). Pour the water over the side of the tank. Next, feel the metal with your hand. Where the water has succeeded in warming the tank, it is empty; where the tank remains cool to the touch, there is still propane inside.