If you see something that causes you concern, you should have your tree inspected immediately.
Otherwise, trees should be inspected every three years or as recommended by a Certified Arborist. In extreme weather conditions like drought or excessive rain, more frequent tree inspections are advisable. Also, consider having your trees inspected after a severe storm, particularly if you have heard cracking sounds or see broken or dangling branches.
Make a list of all of your tree questions and concerns before the inspection so you are sure to get them all answered!