It's easy to spot leaf miners, because they dig little tunnels inside your leaves while they eat. Since they're beneath the surface, pesticides are often ineffectual. The best method of attack is to pinch off the infected leaves where they join the stem, put them in baggies and throw it out. Alternatively, you can follow the little trail on the leaf until you find a tell-tale lump and pinch it between your fingers. A gruesome end for the little guys, but it works. Sometimes you'll even get help from the natural predator of these little vermin - a teeny tiny wasp - which can do some of the dirty work for you.