Symphylan damage looks like pinholes in potatoes and other tubers. Above ground, the plants look stunted. Control of symphylans is tricky, even when commercial growers use soil-based insecticides. The tiny invertebrates move up and down in the soil, away from disturbance. The best way for the home gardener to deal with symphylans is first to verify that they actually have them.
People often mistake symphylans for larger centipedes, which are often beneficial predators.
To find symphylans, sprinkle about a quart of suspect garden topsoil into a bucket of water. The garden symphylans should float on the surface. Or slice a potato in half, put it on your garden soil and cover it with a light proof container that doesn't get too hot in the sun, such as a white PVC end cap. Leave it out for a night or two, then quickly lift the container and examine the potato. If you have symphylans in your soil, the potato should be crawling with the tiny white creatures.