I touched on the fact you can hold a light to it and see how much fuel is left. I have never tried to check the level on a trip with a headlamp, because I am incredibly smart — I check it before I leave home with a flash light. Below is an example.
I often read gear reviews that are really just initial impressions. Some folks use gear for a year or two and call it a long term review. I think if gear is used for 10 or more years, it qualifies as a long term review. This way we can answer the question, “Does it stand the test of time?”
For the past 40 plus years there has been only one piece of gear I have taken on every backpacking trip. It has also accompanied me on most day hikes and camping trips too.