Thanks for reaching out with your question! A good starting point before investing in testing of your tree might be to check this resource:

Key to Common Problems of Dogwood

Here you can find an illustrated guide that shows common dogwood problems and lists the associated symptoms. Do any of these match up to what you have observed with your tree? Once we have a sense of what may be happening to your tree we can help point you additional resources that may be of help or to additional testing if needed.