You could use health score to monitor throttling. The health score determines the health of SharePoint Server on a scale from 0 to 10, where 0 is the most healthy state. SharePoint automatically starts throttling request once the health score is to high. The health score can be calculated by using many parameters, such as memory usage, concurrent requests etc.
Reference:
https://www.wictorwilen.se/blog/sharepoint-2013-sharepoint-health-score-and-throttling-deep-dive
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