How do you know he's just using you to make his ex jealous?

He was smiling that time and he opened his arms widely like telling me to come to him so I did and gave him a warm embrace. We hugged too long and that his friends were just looking at us smiling like actualing seeing a sweet part of a romantic movie. He kissed me twice on my left cheek, haven't heard of that kind of greeting in Italy. I'm pretty sure, you kiss a person cheek to cheek, but instead his lips pecked my left cheek twice. He even introduced a friend and we did talk for a bit because I had to go home... His ex was there and she was actually looking at me when I passed by.

I went out that night because there was an event at the plaza. When he saw me, he said "Here she is..." and he stood up, left his friends and came to me to give me that cute hug I still can't forget. We talked about how he's doing on his job and about me being in school. We haven't seen each other for 3 weeks and it was just nice to feel like he did miss me when he came to hug me right after he saw me. ... His ex was just right behind us.

Okay so maybe you're gonna ask "Does he only do that when his ex is around?" No. There were also times when he'd randomly do that, but honestly when his ex is around, I feel like we hug a little longer.
He always uses the perfume I've given him on his birthday though and I like the thought of that.

After a few weeks, I went to the bar and saw him talking to two people... his ex and a guy. Although I didn't know what was going on, it made me feel sad and the first thing that I thought about was he was just using me to make her jealous. But if i wasn't mistaken, I think I heard him said my name but I didn't want to look. I hate that I felt jealous. After a minute or two, he actually left his ex and the guy and went to his friends.

My best friend said he's unpredictable.
Considering all these things I've written, is he just using me to make his ex jealous?
How do you know he's just using you to make his ex jealous?
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