Ron thinks everyone knew he liked Hermione before he figured it out himself.
He's not quite so clueless now, thank Merlin. He did need a bit of help though.
"Help", defined as every one of the other boys in his dormitory (including Harry, who is due for some serious revenge) ganging up to make it happen.
Neville had asked Hermione to come to the dorm to help him do his Arithmancy homework. At a time when he knew full well Ron would be the only person in there.
When she arrived, the door slammed behind her, and the other four made sure it would be staying closed until they said so.
To give them credit, though, their plot did work. Well, faced with such an effort, what else could Ron and Hermione do?