Look for the male cleaning an area of rock and then watch to see if he sits by it for about 5 days, not moving away except to chase bigger fish out of the area. They can breed at that size.
Clownfish dancing or shimmying is not necissarily a sign of breeding, but more a sign of dominance in the female. This does usually mean that they are mature enough to be showing sexual signs of pairing, but again, does not mean they are breeding.
Before eggs come or when eggs are not visible, the above mentioned signs will be able to give you an idea of when they are about to breed. The male's picking at the rock is the best indicator, because he is specifically doing this for a place for the female to lay eggs. This will not be a process done much except for when the female is about to lay.
Also, I don't know the exact way to describe it, but you can check the female for an enlarge belly and a visible ovipositor and these will be the tell-tale signs of a female ready to lay eggs.
