Well... you just give examples. They could be funny or just something real.

You know you're obsessed with Arashi when...
     The Pizza Hut commercial that advertizes Pizza-mia pizza reminds you of Ninomiya.

→ The first reaction you give when you sit for a very important killer Math paper is = DANGER! BIG DANGER DESU!
→ You go to a ramen store and drink the soup, leaving out the noodles
→ You learn Nihon-go from Arashi's songs and variety shows
→ You ship OTPs
→ You cannot look at a chipmunk straight in the face without thinking of Sho
→ You flail excessively attempting to dance to their music
→ You bought C1000 even though the price is such a bitch for that teeny bottle because Arashi did a CM for it
→ You grin like a dork each time you think of them
→ You pray to Ohno before your Art Examination
→ You pray to Sho before your Maths/History/Language paper
→ You pray to Aiba x Jun before your operation
→ You pray to Nino or go game shopping going, "HEY RPGS! NINO PLAYS THEM :D"
→ You attempt sappy, half-wonked fanfictions even though you suck at them
→ You get affected by hate-memes
→ You cry with them, smile with them, act like an idiot with them
→ You actually pickled a hamburger to see if it tastes like cheeseburgers
→ You did the Yoga stretching thing in Arashi no Shukundai
→ You played the drawing-matching game with your classmates
→ You call random dogs "Atomu/Hime" and call random hamsters "Sen"
→ You have made 938040913422123 variations of Cha-Han
→ Or korokke for that matter
→ You think every small thing they have done is for their fans
→ You just busted your 320GB External HDD on downloads
→ You simply can't believe Ohno's 27

- You rip your pants and think of Sho-kun (just happened to me, I swear xD)
- "Do it" becomes your mantra
- You went to the Gap specifically looking for Aiba's ads
- Sho's belly button piercing makes you want one
- You can predict what Arashi is going to say, even when your Japanese is very limited
- You've started adding 'Maximum' before adjectives

LOL You pray to Sho before your Maths/History/Language paper is so true for me!

- You instantly have a high regard for someone just because you know they like Arashi
- Your house has already memorized Love so Sweet or any other Arashi song for that matter due to your incessant playback.
- You doodle them.
- You have at least one fantasy about them.
- You say 'mecha umai' whenever you taste something delicious.
- You use Arashi as examples to illustrate your point.
- You've attempted to try at least one A no Arashi experiment.
- You see the rainbow as 'Arashi colors'

you're so obsessed with arashi when

- your music player, mp3 songs is flodded with their songs
- everytime an exam come, you pray that the next morning you wake up sho's brain would be yours
- you can talk the whole day about arashi
- you say umai to everything you eat
- you don't mind your boyfriend to be taller or shorter
- the first kanji you memorize is arashi, then all the members name
- you think nino as nemo
- whenever someone say captain or leader you think of ohno
- you day dream all the time
- you want to buy the same games as nino
- you want to own a nitendo blue ds
- you want to try everything that arashi tasted in ANS
- eating wierd food is not crazy
- you want a pet and his name is momo

- you compare every single boy you see to one of the members
- you stalk the boy when he DOES look a bit like a member

I TOTALLY AGREE ON THIS!!!
i've been reading the newspaper everyday after Sho became a newscaster

-your quest to learn japanese was greatly inhibited after you discovered arashi pvs/variety shows/fanvids/bloggs
-your quest to learn japanese has turned into your quest to learn japanese slang (@gewndy - maximum umai!!!  so true i use maximum soo much now )
-the only japanese you can write is 嵐 and you find reasons to write it on everything hoping that someone else will recognize it and say "hey! i like arashi too!"

-you rename all of your Final Fantasy (or any other RPG) characters after the members of arashi