Let me think, hidden identity and main lead who will fall in love with him or her has to find out early on or knows it from beginning. Its hard to tell, a lot of dramas have hidden identity but you can try drama Seven of me- FL knows that he has seven personalites DID, Sparrow- both main lead have hidden identity and they find out it early on, Birth of beauty- he knows it from begining, Chief Kim.
But perception of time is little bit hard for me, i dont really know what is early on. You know for example when my best friend tells me to wait for a while, she means three hours but when i say it, i mean five minutes. So i will add some of my favourite dramas with this topic and i dont remember when they find out. It could be in episode 3, 5 or somewhere half in the story: My little princess, Star April, Zombie detective, Meow the secret boy, My girlfriend is an alien, Precise shot- he knew it from begining only ML with hidden identity didnt know that he knows it, Unexpected.
And it wouldnt be me if i didnt add it: Great man academy- he found out it in last episodes and you can think that its some children story because of unicorn wish but its must watch.