These are the signs that you've outlived your usefulness to the world of technical writing:
10. You translate your spouse's every word into Plain English.
9. It's not the graffiti that bothers you, but the misspelling of "mother."
8. SMEs ask YOU questions.
7. Your name appears on every wikipedia edit page.
6. Upon hearing the Next Big Thing you think "I want to document that!" before thinking "I want one!" (Thx K. Mulholland)
5. 
You carry whiteout and a dictionary next to your lipstick and compact (or in your pocket protector).
4. Reference sites link to you.
3. You actually know how to use a semicolon.
2. You make your kids write a business case for why they should have something (like the latest phone). (Thx B. Graham)
1. You refuse to bookmark this list due to the number of grammatical and punctuation errors.
Bonus sign you've worked in tech writing too long:
OED calls you for suggested definitions of new terms.