If you suffer from heart failure you may develop abnormal breathing patterns. (health.com)
With time, you'll become aware of what your pet normally looks like when breathing and what abnormal breathing would look like. (wvrc.com)
Some dogs will prefer to sit or stand; you might even observe them trying to fall asleep in these positions, and close observation of the breathing effort will often show abnormal breathing in these animals. (vetmed.tufts.edu)