I can't always tell whether my characters will be likable (or likable enough for a reader to follow for hundreds of pages). Many times I don't know where the momentum stalls and the story gets boring. Sometimes I can't even tell if the plot is believable. I'm just too close to the work to see it correctly.
Fortunately, I've got friends who help me out. When I've wrestled with the novel until it's as good as I can make it, I send it out to a handful of beta readers. Their input helps me make the novel even better. I've got a book out with beta readers right now, and I can't wait to get their feedback.