This is one of the classic puzzles of adult social life. Someone smiled at you — was that interest, or do they just smile at everyone? They agreed to your invitation — was that a genuine yes, or did they feel too awkward to decline?
Any single social behavior can't serve as reliable evidence. Someone replies fast — maybe they happened to be holding their phone. Someone laughs at your joke — maybe you're genuinely funny, and it has nothing to do with attraction. A single behavior has too many possible explanations. You need a pattern.
These behaviors carry more weight than single signals: Do they initiate contact on their own? Do they remember details you mentioned before? Do they treat plans with you as a real commitment? Do they offer alternatives when they cancel? Especially — do they reach out when there's no practical reason to?
Another valuable signal: Do they interact with you outside of group settings? If all your exchanges happen with mutual friends present, it's far more likely to be ordinary social friendliness, not romantic interest.
Offer a low-pressure, no-cost way to decline. For example: "If you're free this weekend, want to grab coffee? Totally fine if you're busy." A genuinely interested person will accept or propose an alternative time. A merely polite person may accept and cancel later — or not respond at all.