There's some more nuance thank you think to it. How long have you've been here?
This is a gross oversimplification, but it should be enough for you.
Generally, if a girl is pubescent and has certain secondary sex characteristics (e.g. wide hips, slightly engorged breasts, and many others), she is not a loli. Look up "Ladylee Tangleroad" from Toaru Majutsu no Index, or Yotsubato from Yotsuba to see how a loli's body type looks like. No feminine wide hips, no breasts. If you gave them a haircut and made her wear boys' clothes, they would be pretty much indistinguishable from a boy in appearance. That's what a loli is.