The phrase means trust the evidence of someone’s behavior over their words, and take them at their word the first time you notice who they are. Key ideas

• Actions reveal character: What a person does in real situations shows their values and priorities far more reliably than what they say.
• First impression matters: When someone demonstrates their true nature, it’s often best to believe them rather than rationalize or give chances after chances.
• Cautions and nuance: People can grow or change, and context matters. The saying encourages awareness and boundary-setting rather than cynicism.

Practical takeaways

• Observe consistency: Look for patterns across different situations, not a single incident.
• Pay attention to red flags: When behavior contradicts stated values, treat it as meaningful information.
• Protect your well-being: Use what you learn to make informed decisions about trust, boundaries, and whether to invest in the relationship.

If you’d like, I can tailor these ideas to a specific person or situation you have in mind and suggest concrete steps for evaluating them.