Stand near the trunk of your tree and look up into the canopy. Crown thinning and dying branches are often the indicator of a GSOB infestation. A healthy Oak will exhibit full thick leaves at the top, but as infestation occurs and progresses, branches and leaves will begin to die and thin. Another beetle (usually the western oak bark beetle) attacks drought stressed trees at in the upper branches. 

Learning to differentiate details about your oak's canopy and trunk are key to evaluating your trees. Doing this at least annually allows one to have enough time to respond and best care for your oaks, before the infestation becomes critical to the tree's health.