Start weighing her regularly. If she is laying internally she will steadily increase in weight. I'm not sure what weight a yolk is but must be a good ounce, so if she is increasing in weight by 3-4 oz a week or more, you probably have an internal layer. Your next step depends on your goals and finances. If she is just for egg production, then you cull her, if she is a pet and you have the finances to spend on her, a vet can give her an hormonal implant(superlorin) to prevent her from ovulating, but it will need to be replaced every 3-6 months and is not cheap. Third option is to try to induce a moult. As I said, moulting season is nearly upon us and she should stop ovulating(releasing eggs) once she starts moulting, which is triggered n part by the shortening of the days, so if you keep your girl in the dark for the majority of a few days, maybe even a week, just letting her out for a few hours a day and limiting her feed, you may be able to start her moulting or possibly trigger broodiness, especially if you give her some fake eggs in her dark prison, both of which would stop her ovaries shedding eggs. I had a girl last summer that was very swollen with it and walking like a penguin, but during the autumn and winter she improved dramatically, only to get worse in the spring when her comb got bright red again and she must have started ovulating again. Unfortunately I didn't manage to save her because it had gone too far by the time I did my research and understood enough about what was happening. Hopefully if this is your girl's problem, you will be able to take action and prevent it getting too bad. Unfortunately she is unlikely to ever lay again normally if this is the problem and you are going to have to keep her in the dark for longer periods through the late winter, spring and summer, to try to prevent her from starting up again, but she should be fine to be out normally with the other girls once she starts to moult for at least a couple of months.