I know most people seem to look for the ovys, but I don't try to catch them. I pay more attention to sheds, if they are eating, and body shape. If they haven't been eating and they shed, I start to watch their behavior to see if they look like they are going to lay. Their bellies also get very round across the belly scales, rather than flat, when you hold them in the air with about 2/3 of their body hanging down (this begins prior to the ovy, just gets more pronounced after the ovy).
Hope this helps,
John