When I was a commercial poultry-man and hatchery I never thought much about checking eggs beyond standard candling periods. This was due to volume in the hatchery and a need to maintain standardized times and practices. Now working to preserve heritage, rare and endangered species I find my methods and thinking processes far different since every chick becomes precious and important to preserve the breed. A decade ago I started using a physician's stethoscope to listen to all eggs that go past 21 days to determine if the chick is still alive and act according to help the chick out if necessary. I purchased a professional high quality tool with soft rubber v cones instead of the typical flat receiver we all know from the doctor's office. The v cone was designed for checking infants and pregnant mothers since the heartbeat is more faint. The rubber allows a tight seal around the egg and provides a more clear sound. Now you can purchase a stethoscope in most drug stores today for under $20. I would imagine this would work fine too and perhaps using a little hot glue around the edge of the stethoscope would make a better seal against the egg if it doesn't work on the flat. Cheers!