I doubt someone can really tell that difference. In order to create a relationship of any kind you have to think of you two as one, so some of his/her features would natually get lost. So the only way I think of it's questioning: would you still like that person if you could never see him/her again? But that doesn't solve much anyway.
Liking someone is accepting them and loving them as they are, and wanting to be so deeply involved in their life that you'll never crawl back out.
Liking the idea of being with someone leaves you comfortable. That's about it.
You are playing with fire
The latter is more confusing because you always have to consider the other person.
Knowledge of that person.