Be slow to trust and quick to notice. The noticing does the protecting.

Suspicion and attention are different things, and only one of them costs you anything. Watchfulness lets you see a pattern early, which is what actually protects you, while suspicion mostly makes you tired and tells you very little. You can be warm and still be paying attention.

Give new people your friendliness freely and your reliance slowly. Watch how they treat people who cannot do anything for them, and how they handle being told no. Let trust grow at the speed of evidence rather than the speed of wanting company.