It's a question that we often get in the shop and unfortunately, there is no great way to tell how much you have. There is one SIMPLE way to tell how much you have, without any expense involved. Although it is just a rough indication.
Step 1 - Turn off gas flow, disconnect bottle from appliance.
Step 2 - Boil some water, pour down the side of gas bottle.
Step 3 - Run your hand from top to bottom of the gas bottle until you can feel a noticeable change in temperature (cooler).
Step 4 - Where you began to feel a change in temperature, this is the level of your gas.
Step 5 - Depending on how far down this temperature change will occur. If you imagine the bottle as a glass of water. If its right up the top, your bottle is full. 1/2 way down, your cylinder is half full.
You can also watch the video linked above and see how easy it is!