In my experience, females tend to make a sort of nest in the laying substrate, sometimes burrowing to the bottom of the laying box. I check my girls once every 24 hours, like Matthew said.
If you're using a clear/semi clear container and you can't see anything from above or the side, then you could gently and carefully lift it up (don't tilt it) so you can check from underneath.
If you're using the coco stuff on the bottom of the container, with a layer of moss over the top, then you could open the lid gently, then lift the moss carefully to check. Try to disturb her as little as possible and if you find her in the process of laying, then leave her well alone for a good few hours before checking again.
Hope that helps, good luck,