Her behavior really isn't all that important if you are trying to gauge their loyalty. All women are always on the look-out for a better option based on what she feels she needs. A better indicator of loyalty is her social conditioning and her past behavior. If you are with a chick that's had a lot of boyfriends, you likely have a chick that is not very loyal. If you are with a chick that comes from divorce, where she was not raised with her father, this is another high risk factor. They all have the trait that instinctually draws them to monkey branching, some chicks, maybe 40%, are capable of overcoming this urge in the interest of reason. But many of these will exhibit b!tchiness and letting herself go if you fail to deliver what she wants, this is her way of punishing her man for not meeting her emotional needs.
If you are NOT delivering emotional stimulation to your LTR chick, she will get bored and shake things up...
@LARaiders85 is right, b!tchiness is a indicator of loyalty.... I can't tell you how many times I heard a story from a friend that said, "Man, I don't know what happened.... sure we had problems but things were getting BETTER, she had stopped all the nagging and b!tching and things were going well. Then out of no where, she dumped me and a couple of weeks later, she posting pics on Instagram with her new BF."
She stoppped nagging because she didn't give a fvck about the dude she is with, and found a better, higher branch. When she is nagging her man she is either trying to change his behavior to something she wants, or she's trying to drive you off. Either way she wins in her mind.