Grape mealybugs usually stay out of sight, and you can find them under the bark.
Mealybugs are often escorted and protected by ants. Finding ants on you vines can be an indication of mealybugs in the area.

Pest control can be done using a systemic insecticide applied via the irrigation system, managing the ant population, and placing fermone lures which confuses the males so they cannot find the females.