If she's got an egg stuck, you should be able to insert your fingertip into her vent (lube, please) and you'll feel it.
Treatment is a soak in warm water mixed with Epsom salt. There's no dose.... just pour some in. Submerge her lower half and let her enjoy the chicken spa.
The magnesium will be absorbed and help her muscles relax. She will hopefully pass the egg on her own after a 10-15 min soak.
If she still doesn't pass it, you can insert a syringe and needle into the egg, suck the inside out, gently crunch the egg up, and (with lots of lube) remove the shell.
I hope she passes it on her own if she's got anything stuck!
MrsB