The only reliable way to sex them that I know of is to look at the breeding tubes when they are visible. You'd probably be better off getting a group of say 6 and waiting to see which form a pair and then rehome the others. The female will have a tube that (for want of a better description) resembles the eraser end of a pencil. The males tube will be smaller and pointed like the tip of a pencil. The female will lay eggs on a suitable site (slate, rock, large leaf etc) and the male will follow behind touching each egg with his tube to fertilise them.