Flirting... in Thoughts on thoughts

So I guess I’m either really Nieve or just stupid… but I think a co worker is flirting with me but also.. I could just be overthinking things.. He talks to another co worker like a sister but with me he goes a bit more.. again.. I could be over thinking this because I don’t really know if I’m being flirted with until someone tells me its happening!! HAHA…
I just need to know I guess… how do you know when a guy is flirting with you?!