Question

What observations will lead to the conclusion that the carbon dioxide realised during respiration

Answer 1

there is an experiment to prove the above mentioned:
exhale some of your breath in a test-tube containing lime water. Do this using a straw so that the exhaled air moved to required direction. It will be observed that the lime water turns milky in colour.
it is the characteristic property of carbondioxide to turn lime water milky.
thus this simple experiment's observation would provide aa proof that CO2 is released during respiration.