Well I’ve never done it myself but white mycelium would form a kind of solid mass between all the rice grains. It might be hard to tell white on white. But it would feel solid, like requiring some effort to crumble. At warm temperatures (75-80F) I would imagine about 3-4 weeks would be sufficient. At cooler temperatures I don’t know. Usually you’d find out that you inoculated some kind of mold, but it is possible the spores didn’t make it in or they were unviable. But from mycelium, then there are a variety of techniques to get the mycelium to fruit, typically involving a new food source and a light source.