It's pretty easy to tell. Snakes should shed entirely in one single piece. If they do not, they're having trouble.
General rule is to increase the humidity in the entire enclosure, not just the hides. Make sure she always has fresh water, some snakes won't drink water that has stagnated in a dish for a few days. When you notice her go into "blue" - those first few days before shedding when her eyes milk over, mist the cage more heavily, and try soaking her in tepid water for 10-15 minutes a day. Double check the temperature of the area over the
heating pad, make sure it's not getting too hot.
There is a fine line between humid and wet though, and sometimes it can be hard keeping the humidity up without making the cage actually damp. You don't want her to be sitting in actual wetness for a long period of time, that can lead to scale rot. This time of year, when most people are running their home heaters, the humidity is quickly sucked right out of the air. The room she is in may benefit from a humidifier.