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. Symphylans are white to cream-colored, have six to 12 pairs of legs, depending on their level of maturity.
Centipedes often have more legs.
Symphylans have relatively long antennae that are constantly tapping the soil in front of them. They move extremely fast for a non-predator, always fleeing away from light. Once you have verified the presence of symphylans, the most effective remedy is to till your garden soil well next spring, recommend Berry and Peachey. This breaks down the runways in the soil and the symphylans can't move around. Avoid planting seeds too early in the spring.