When is it okay to assume that someone is interested in you?

It's way easy especially if that person is a female.

Most of them will reveal the following features

Smiles for just nothing while avoiding a direct eye contact with you.
Initiate chats and sometime even calls you to say hello.
Accept to go on consecutive dates.
Buys you some gifts.
Talk to people how good of a person you are. ( whether she is right about it or not)
Wish to take part in your hobbies. They'll offer to volunteer with you, travel , hit the gym, swim etc.

On the side of men, it's quite challenging . Simply put, men rather see themselves as emotional rocks and seldom will they disclose their affectionate behaviours.

A man interested with a woman at some point presents as one who isn't interested but pretends to.

However, a man who is interested in a woman will show the following features;

Patient enough and he shows it in a less strained fashion.
Tells his fellow friends how he sincerely likes you.
Defends you , even when you're losing it.
Respects your choices with no interrogations.
Slows down his pace to match yours while the two of you are walking together
Sacrifices what he likes ( watching football matches since this is something most men like) and joins you for a concert, rally, volunteering, game, movie night etc…
Remembers tiny details; like the food ingredients you hate, your birthday, size of your clothes, your appointment schedules( ofcourse ones you shared with him), your favourite dress codes..

I hope that helped.