How do you know if someone is flirting?

[Anonymous]

There's this guy at work.

I dont know if he likes me or is just being friendly.

I had to ask him a work question on Friday, he had to look at my screen & he was leaning really close to me.

In the morning & night he has to walk past my desk so he usually says good morning & good night & waves

On his way home tonight he did his waving thing then he stopped & smiled at me for what seemed like a while.

I don't know if I'm over thinking it.

How do I flirt back with him to see if he's keen?

[quaranta]

This is not enough to call it flirting, it seems like he is just being nice and maybe he is an extrovert so greeting is spontaneous and he maybe sees no problem on leaning on your personal space.
You should get some jokes that indicate some interest, or that are aimed to check if you are married already, or some personal contact where he tries to get to know you better, some touches in a playful way, to actually call it "flirt" with clear signs of potential (potential) interest.
So create some occasions to talk with him more, or one-on-one for what looks like a coincidence, like during lunch break, and be caring and easy to talk with, but also simple and accessible, positive in general. That should give him less hesitation if he has any. The more you have chances to interact with him the more you will collect data, but be very aware to not see things where it's just "being kind" and especially don't build up fantasies on top of the potential nothing, wait for inputs to keep it realistic and not hopelessly crushing on him.
Also keep in mind that some men just like to flirt "as it is", with no plan behind. Not wanting to date, or a relationship, or even a hookup, but just flirting only. And that is not even a constant desire most of the times. So if you want a definitive answer you need more clear data for sure.

[midnightcavern01]

Probably just familiar with you. You're reading into it

[Guy13]

We are never friendly and that’s why opposite sex friendship does not work. There’s intention but he has to be greater than 50% sure you are not turning him down.

[serious]

I cannot see any clear signs of him being flirty.

[In_Trance]

pay attention. you'll see