Something I didn’t learn until relatively recently was that they didn’t show Yoda in any of the pre-release ads in order to leave his appearance a surprise.
I can’t say for sure if I would’ve figured out that the little green creature was the great Yoda, it probably would depend on what age we’re talking (I can’t recall a “first time” viewing of the original movies, I feel like I was born having already seen them). But I think I would’ve had some inkling I was in for some kind of twist, seeing how peculiar a place Dagobah as and how mischievous and tricky Yoda appeared to be. And if I were aware of any of the ads or extra material for the movies I think I’d find it odd that I hadn’t yet seen any person or figure who I could peg for another great Jedi master.
Come to think of it, if I hadn’t figured it out by the time Luke is in Yoda’s home, I think I’d probably find the scene profoundly sad, seeing this little old creature who is obviously stalling for time because he’s all alone and never gets any visitors, lol.