Sometimes the second crack is very slight. With the hot air popper I use it's not subtle at all and is very distinctive. With my Fresh Roast I have to listen closely. Generally the first crack is very noticeable and you hear slower and louder "pops", or cracks. The second crack is quieter and faster, sort of like Rice Crispies. If you have trouble hearing the second crack, watch for color changes. IMO, once the beans turn shiny (oily), they're overdone. Some people like certain beans this way but only just shiny and not any more than that or they're burned. Another thing to look for is smoke. When the beans just start to smoke, they're done.