It used to be that a chimney inspection consisted of whatever a self-described chimney inspector thought was necessary. Or, if the inspector was less scrupulous, it consisted of the least he thought he could get away with. The National Fire Protection Association (NFPA) has stepped in to rectify this situation. They have proposed three levels of chimney inspection and have given details on what service ought to be provided at each level.
This is the most basic inspection. If no changes have been made to the chimney and you don't plan to change the way you use it, a Level 1 inspection should be sufficient. It's a basic checkup to make sure that things are still running smoothly and there aren't any obvious, imminent problems.
A Level 1 inspection should involve an examination of the "readily accessible" and "accessible" portions of the fireplace and its connection to the chimney. The "readily accessible" portions are defined as those parts that are exposed or capable of being exposed without the use of tools. For example, the firebox, damper, and the lowest part of the chimney where a flashlight can be shined would qualify. The "accessible" portions are defined as those parts that can be exposed using commonly available tools (hammer, screwdriver, etc.) without damaging the chimney or building structure or hurting the finish.
If you are making changes to the fireplace system, a Level 2 inspection is in order. These changes can include switching from gas to wood as a fuel (or vice versa), altering the shape or material of the flue, or replacing or adding any appliance of a different type, input rating, or efficiency.
Building fires, chimney fires, and earthquakes can all warrant a Level 2 inspection as well.
The inspection should include everything listed for a Level 1 inspection. Additionally, it will check for proper clearance of all combustible materials in accessible locations. And it must involve examining accessible portions of the chimney, both outside above the roof, and inside where accessible through attic crawl spaces and such.
A Level 3 inspection is warranted when a Level 1 or 2 inspection suggests there are hidden hazards that cannot be evaluated using the tools at hand. This inspection will remove or even destroy those portions of the appliance or its surroundings that prevent the free inspection of the potentially affected area. This often includes removal of a chimney crown or an interior wall, but it can include more severe measures as well. Because of its nature, a Level 3 inspection is never the first option. It is only done when lower level inspections indicate that it is necessary.