The Inquisitive Belle...

Good question. As a kid, I thought that she must have suspected that the Beast was the young man in the portrait in the West Wing. Otherwise, the movie never actually brings this question up later, despite how she obviously realizes how the castle is enchanted.

I'm not sure what was going through her mind when she looked at the torn-up painting, and to be fair, it had been slashed up so bad by the Beast that I doubt she could make out much. She probably had a lot of questions going through her head as to why this creature was living in this castle, where he came from, why he has magical objects for servants, etc. I'm not sure if she figured out he was under a curse or not, for she didn't seem to pick up on it, nor did she seem to figure out until after it was broken.

While Belle was well-read, you can see from early scenes that she didn't have access to a ton of books like we do today, so she only had so much to feed her imagination. While she was an intelligent character, even intelligent people can sometimes miss what's going on right in front of them, particularly if they're distracted by a lot of things going on in their lives. It's one reason some people aren't even aware they're developing feelings for someone they interact with all the time, but everybody else can see it.