You don’t realize it’s the last breath until the next one never happens. At least, that has been my experience when watching someone die, pass away or, in medical terms, expire.
We typically don’t think about breathing, either our own or that of others. It’s similar to the effect one gets when there is a power failure in the middle of the night–you’re alerted to the power failure by the sudden silence. All those hums and rumbles from refrigerators, furnaces, and humidifiers abruptly shut down, and the silence is audible. This has been my experience when patients take their last breath.