Robin, thank you for your question to "Ask Extension".  Most galls are caused by irritation and/or stimulation of plant cells due to feeding or egg-laying by insects such as aphids, midges, wasps, or mites. Some galls are the result of infections by bacteria, fungi, or nematodes and are difficult to tell apart from insect-caused galls. Seeing the insect or its eggs may help you tell an insect gall from a gall caused by other organisms.  With that said,  you could remove a gall and bisect it to observe if there are any eggs or larvae.  If there are growth rings then it is living tissue.  And if it is just soft tissue it was probably caused by a feeding insect.  Plant gall damage is usually an aesthetic problem and is not considered serious. Affected trees ordinarily show little injury and once the gall begins to form the process cannot be stopped.  Treatment is not usually recommended.
     I hope that you find this information useful and as always Happy Gardening.