DD is only 2, but when we were exiting therapy, I asked the developmental therapist 'how do I know if she has legitimate oral needs or if it's reached a point where it needs to be stopped?'. Her suggestion was to purchase a child chew toy designed to stimulate the oral sensors that some children need. I'm not to the point where I am doing that, BUT, she did say that instead of taking everything out of her mouth, I should offer an alternative. So, I now allow her to chew on one of her old rubber baby spoons. But everything else gets removed from her mouth.
So that's a good starting point. Give her one thing she's allowed to mouth and see if you can contain it to that.
And you can google sensory tests and just see how she scores on the oral to see if there's a reason for additional concern.
SAHM to Magnolia May (09/10) and Luke Russett (04/13) and wife and best friend to my airman.