As said above. They will usually stay in the same position for up to a few days. Also, the larger the specimens, you will notice the wing buds slightly puff up. This is espeically noticable on the sub adult specimens.
In my experience, it's quite unpredictable. Sometimes, they have a really large meal in the evening and I find them molted in the morning. Other times, when they don't eat, they don't molt and then die. Yes, and other times, when they don't eat, they molt a few days later.
But what are the signs? I've heard that they hang upside down, but all the mantids I've ever had have done that all that constantly and they were adult mantids.
I watched two mantises molt to adult. Right before they molt, they hang upside down. They're a little clumsy at this point so they might fall. Anyway, their breathing becomes a little different. Their abdomen looks like they're taking deep, slow breaths. I think it's them moving their insides. And then, molting will proceed soon.