While I figure I might as well create one of these lists, seeing as to how popular lists are and the large number of these lists that had been compiled in the past.
1. You can point out translation errors in the manga/anime.
2. You scourer the internet for Ranma related things.
3. You remember small insignificant details like Kuno's dog name being Arumajiro (Armadillo).
4. You dream about the characters of Ranma ½.
5. You create a website on Ranma ½.
6. You make a detailed analysis on the Manga (like # of times Ranma use hand gestures or times Akane hit Ranma).
7. You spend hours arguing with someone else on the details of Ranma or how he would fair in a fight versus someone else.
8. You write a fanfic that if printed would be around a thousand pages long.
9. You've read the manga in multiple languages to compare the differences.
10. You call people baka and hit them when they upset you.
For more see these sites:
You know you watch/read too much Ranma 1/2 when geocities site.
Dragon Len list (page background gone highlight entire page to see).
Anime Info Ranma 1/2 Library