On a Mac, running applications are represented in the Dock with a tiny indicator light beneath the icons that represent said applications. iPhones and iPads also have application Docks, but no such indicator appears on those platforms.
Whether or not such a feature would be useful on an iPhone or iPad remains to be seen, however a newly released and free jailbreak tweak called Dock Indicators by iOS developer LacertosusDeus can bring this functionality to these devices in a fun and customizable fashion.