What is the single most reliable indicator that an individual truly understands the Stoic 'dichotomy of control'?

The single most reliable indicator that an individual truly understands the Stoic 'dichotomy of control' – the understanding that some things are in our control (our thoughts and actions) and some are not (external events and other people's behavior) – is their consistent emotional response to external events. Specifically, the absence of prolonged negative emotions like anger, anxiety, or resentment when faced with uncontrollable external events. Someone who understands this principle doesn't just intellectually acknowledge it; they embody it by accepting what they cannot change without significant emotional disturbance. For instance, if someone is stuck in traffic (an uncontrollable event), a Stoic practitioner wouldn't become enraged or intensely stressed; instead, they would accept the situation and focus on what they can control, such as their reaction to the delay. This emotional equanimity, this persistent calmness rooted in the acceptance of external realities, is the clearest sign of mastery.