Todomomo

Not subtle at all. Momo would blush everytime he walked past her, bite her pencil, and she got all happy when he was around.

For shoto they couldn't tell at all. The only queues they had were him blushing everytime someone mentioned her.

Kirimina

Kiri told them.

Mina bit her lip around him, would stare off into space, and would stutter a bit.

Miruhawks

Ryuko is just good at reading people. Miruko can bottle it up pretty well but she slipped up once as well and while they were talking about something dumb, Rumi said his name more than once and Ryuko said

Hawks just said "yo Miruko is adorable and I love her."

Iidamei

Mei's friends just saw how some things wouldn't blow up like she was trying to make them perfect for Iida.

Iida would slip up and start stuttering whenever he saw her.

Kamijirou

Kami can't hide that at all.

Jirou was more tsundere-ish around him.

Dabi x Toga

Shiggi: Why tf are you staring at each other?
DabiToga: we're dating-

Nejitama

Mirio didn't, he just thought they were cute and got his girlfriend to cheer them on.

Mt.Kamui

:/ it was kinda obvious they had caught feelings.

Ryushot

Rumi listened her drunk rants about how much she loved Shinya, wanted to marry him, and have a family with him and then watched her cry because of how lonely she was. It wasn't pretty. (AkanoKaguya522 👀 bet you won't write it.)

They literally didn't, Shinya knows how to keep it buried and knows how to do it very well. 

HitoRei

Reiko and him spent more time together, she started smiling more, and she just seemed happy.

Denki is just an idiot and took a wild guess that ended up being right.

Tetsukendou

They said "he/she is cute"

SetSero

Setsuna kept murmuring vines about dating under her breath.

Sero would daydream in the middle of class. And draw pictures of her.