Experts have come up with a clip on disk that tells you to sit up straight and breathe through your belly.

The app can work with most Bluetooth-enabled smartphones and also offers breathing exercises from all around the world, including yoga pranayama, tai chi, and buteyko, which can be organized by time of day such as "Wake Up" or "De-stress."
Prana learns how you breathe over time and then makes recommendations based on what you are doing specifically. The Prana Clinical Mode offers either you or your health practitioner data on how well (or poorly) you are breathing while sitting, standing and exercising throughout the day. Practitioners can also see if you are sitting upright or mostly slouching to make an assessment.
Persidsky said that they wanted to offer a truer, more holistic picture of breathing quality, which necessitated factoring in posture, but also providing the capability to distinguish between chest and belly breathing, as most traditions of good breathing (and recent science) emphasize the value of breathing from the abdomen.
Source-ANI