Practice Drill #1: Write your resume. List all your relevant skills, then note the ones that will still be needed in 100 years. Give yourself a 1-10 rating in each skill.
This drill will help you see where you need practice. It won't turn up your "blind spots" -- i.e., areas that you don't know anything about (hence aren't on your resume) but that you should know something about. But it'll at least help you see how current your working skillset is, and how long you expect it to stay current.
math, computer science, writing, and people skills are for the most part timeless, universal skills even after 100 years also, it will be useful. Most specific technologies, languages and protocols eventually expire, to be replaced by better alternatives.