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, is the level of your gas.
Step 5 - Gas, when stored in the bottle is liquid and will absorb the heat from the water quicker than the empty portion of the gas bottle. ie - half way down, it's half full. 2/3 its 1/3 full.
You can also watch this video, to see how easy it is!
Want to know something, but cant find the answer? Ask me, I'll try find it for you!