You will know that Solanaceae likes you. They will spend time with you. They'll take time out of their day to help you with their problems. Usually with... oddly prescient knowledge of what's bothering you, but they're genuinely willing to listen.
You probably get the impression that while they've heard a lot of this before...

They're never bored by you. Or unwilling to pay attention.

Also they're probably going to flirt with you at some point unless you've indicated a lack of interest.

Also the gifts. Dear god, the gifts. Constantly. Whether it's a special occasion or not. Weird shit, too, just like. Rocks and seeds and eggs they found out in the world? This pretty rock is the same color as your eyes, it made them think of you, they know you love fruit so they brought you some wild edibles, etc

(and they always remember your birthday)

Akechi's... 'affection' is more subtle.

He'll spend time around you, silent and engaged in His Own Thing but still present. He'll argue with you- all the god damn time- but over time his tone will get less annoyed and more... confident. Almost playful. Like this is just fencing and he's enjoying himself.
(Because he is)

at some point he'll probably just start unconsciously helping you with shit

somewhere along the way you realized that you're probably one of goro akechi's closest friends
whoops.