First she has to be sexually mature. She will show mustard yellow splotches when she reaches sexually maturity. Then she will likely show non-receptive coloration (dark background with bright turquoise spots) indicating that she is working on eggs. It's hard to know when she will lay infertile eggs which is why I suggested having an appropriate place in her cage for her to dig.
Here's just about everything you'll need to know about owning a female, egg-laying species chameleon. Sorry to say but yes veiled's will lay eggs without being exposed to a male. They lay infertile eggs just like chickens.