some of these are from personal experience

1. request the DJ to play some BTS music. If he/she plays it, he/she may be an ARMY. If other kids start to butcher Korean lyrics with you, they're probably ARMYs, too!

2. wear ALL the BTS related stuff you can. if they say, "is that BTS?" or "is that <insert member here>?", than they're most likely an ARMY

3. yell out fanchants at recess/during gym class/school dance. the people that start yelling with you are ARMYs

4. BTS quotes. quote some funny things BTS has said, like Jungkook's "International Playboy" or Yoongi's "Everyone so weird today."

5. uhh...just ask ig

6. write on the classroom whiteboard. put some BTS related stuff on there. People that understand what's on the board understand you :') (jk but try to befriend them)

7. screech "i lOVE BTS!" hopefully someone screeches "i lOVE THEM TOO!" back :D

that's all I've got :/