Photo: Derek Ramsey, Wikimedia Commons
Flower buds typically appear before leaf buds on dogwoods, and are rounder in appearance than leaf buds.
You should start reporting "yes" to the phenophase Flowers or flower buds when the flower bud that is present throughout the winter starts to expand its bud scales and show the flower buds inside.
Also, remember that the "flowers" you are looking for are actually the small yellow/green flowers inside the large white bracts. Do not report a "yes" to Open flowers until you see the small yellow/green flowers open to show their reproductive parts.