Use hand signals. Hand signals tell motorists what you intend to do. Signal as a matter of law, courtesy, and self protection. Scan the road behind. Learn to look back over your shoulder without losing your balance or swerving left.
Some riders use rear-view mirrors.
Watch your speed. There is now a 15 MPH speed limit on all King County trails. Do not cut corners. Respect your safety and others, don't speed. Ride at reasonable speeds.