It's hinted many times throughout the Dollhouse series that no one ever returns from The Attic. Unfortunately, Echo gets sent there, and we learn she was sent there "undercover" to get information and get back out.
But if no one gets out of the Attic, how do we know that anything after she was put into the Attic is true, and not a dream? How do we know that she knows to fake being dead to escape the hallway? How does she know to die to get out?
There are many indications throughout the series that Echo/Caroline is special (and was only recruited into the dollhouse) because she could resist being wiped.
She was the key to Rossum's plans because they didn't want to develop wiping as a mainstream weapon until they had a defense against it - a defense they could create from her.
When a doll is wiped they are stripped of their personality and put into a "Doll State", it's not unreasonable to assume that the Attic involves a similar set of technologies to plug them in (all very matrix).
If Caroline has a resistance to the wiping process it's not unreasonable to assume she has a similar resilience to the techniques used to install someone in the attic. This allowed her to escape and free Victor and Sierra.
As do how do we know whether she escaped or not? Impossible to say for sure, only that her experience in what we know for sure was the attic seemed very different to outside. People in the attic are kept in a constant state of terror, while the end of the show was fairly fast paced it had a happy ending (well, until the Epitaph episodes!)
