How can i know if he is interested in me

First of all i want to appologize about my english that is not very good, second i want to thank you for taking the time for reading this. And if you can give me some advice about this i would really appreciate it.
The thing is, im in love with this guy from work, Alex. Im 29 and he is 37. When i started working in this shop i didnt pay much attetion to him, he is not really my tipe. Latetly i cant stop thinking about him.
Im going to try to explain our relashionship and it will help me a lot if you guys tell me what you think. Is he just playing or is interested in me?
He is very outgoing and he always tell me i look good, he stares at me, he smiles at me, he tells me he wants to marry me, and he makes me feel so nervous but i dont show it. i laugh and i tell him ok, leave, or bye.
We watch movies together when we have time. He helps me when i need help and i help him.
But a couple days ago i felt he act a little bit different with me. The other day he was upset because i didnt say hello to him, then he waited for me outside my office and he hugged me and gave me kisses on my neck. I pushed him away and left.
He told me im very provocative and he is not made of stone.
Sometimes he ignores me, i can be right beside him and he ignores me completly.
the other day he came to my office with his friend (martin) and he told martin im very beautiful, martin asked me why dont i give alex a chance since we are both single. Alex asked me to go to his house for dinner. And i tell him no. I was soo nervious i couldnt take my eyes off of my computer.
Later that day martin told me: alex is a good looking guy why dont you like him, if i were i girl i could date him. After saying that alex came and i told him that i think martin is in love with him and he told me: too bad because im in love with you. i laughed and told him: not your not. From that moment he was very distant. he wants me to say hello to him but he never say goodbye when he leaves. I dont know what to think, how can i give him a chance if i dont know if he is serious. I dont want to get hurt, i really like him.
Thanks for reading this, and again sorry about my english.

He is definitely interested in you. The fact that he smiles and stares at you all the time, as you say, is just the proof.

Or ... Shall I say, he "was"?

Clearly, you refusing all his advances made him think that you are not interested at all. I hate to say it, but it's your fault for him rejecting/ignoring you. He tried, but you seemed to be nervous or not want, always saying no. He is serious, trust me, because you wouldn't invite someone to your house for a dinner, then kiss after work, if you were to flirt. Give him a chance!

Call him, tell him that you are interested and would like to be with him, or I don't know ... You tell him what you think!

Thank u so much. I really appreciate you being honest. And i think you are totally right. Is my falt im too cold with him and not withthe other guys but is just because i like him. Im afraid he wants just sex or he is just joking and i could look like and idiot if make a move. So im really confused. Thanks again for your response.