What the previous two gentlemen has said is true. Under Part 135 it is a little bit different. You don't have to be a CFI to be a check airman or instructor, but you must meet all the training requirements thereof. You must be knowledgeable of the aircraft, company manuals and procedures and regulations. I am not sure how it is under 121, but under 135, you must be approved by the company's Principle Operations Inspector. When approved, you will be issued a letter stating that, containing your authorizations and limitations. Plus a copy will be place in your pilot file.
It can be fun. You get to do things on a regular basis that you normally do just during annual training. It does tend to keep you sharp. The down side, more work and responsibility. More face time with the Feds. A lot more paperwork.