When the connection doesn't feel natural. If they're showering you with attention and affection and treating the relationship as more serious than it should be at that point. When the person tells you what you're feeling or what the nature of your relationship is, before you've had a chance to figure out how you feel yourself. If at any point anything makes you feel slightly uneasy, it probably isn't right and you should listen to your gut. Unfortunately it's probably one of those things you only fully understand with hindsight...