Girls will make it super obvious that they are interested in you, at least most of the time.
They'll get close to you. They'll smile a lot and laugh when it doesn't make any sense at all (you didn't even tell a joke).They may even hold your hand or touch your shoulders unprovoked. They'll keep up the conversation even if you don't seem to say much and seem disinterested. They'll ask you a lot of questions about yourself and will act really happy if you even have the slightest thing in common. They may even find ways of complimenting you. These are the less super obvious, but still somewhat obvious ones. If you're hot enough, she may say something like "hey handsome" or "you're hot" (words an incel will never hear in his lifetime).
"She smiled at me" is not an IOI. Even a single IOI alone, unless it's a very obvious one is not worth anything and mean just about anything.