It's been more than a few years since my CFI expired.
I know that I do NOT have to take a written but I WILL have to take a check ride with a Designated Examiner or the FAA.
So what should I study to knock the cobwebs out of my brain?
rfame made an excellent suggestion. However, if that is impractical in your situation, let me cover a few points that most instructors don't understand.
(1) You only have to take a check in ONE of your ratings. For example, if you have a CFI in Gliders, ASE, AME, and Instrument-Airplane, you can pick from among the four different PTS's and just do that one.
(2) The examiner is only required to test you on a part of the PTS, not the whole thing. Look for an "Additional Rating Table" in the PTS you select. You will see that there are many items the examiner need not test. The most important of those is there is no need to test you on the FOI stuff. The examiner can test you on anything in the PTS, but I have yet to see an examiner test on more than the required items (except through error, ignorance, or circumstance