How can I Prove He's Cheating...

I'm going to keep this short (if I can LOL)

He was caught cheating earlier this year. After a deep talk with myself and later with him, I decided to forgive him.

Well, we're now officially engaged as of a week ago.

Yesterday, we needed to make a stop by Walmart because my stepdaughter ordered a package online. I needed to get a Christmas gift idea so I walked away for a second. He was in line at customer service. I saw what I needed and decided to wait to buy it (will go back when I'm alone) and as I am walking back to customer service, he's texting. No big deal. He was so deep in the text that he didn't see or hear me when I walked up. I said "Hey, I found what I needed." He didn't respond and I was like whatever, he'll answer eventually. I happen to glance down and saw that some woman sent him a selfie. He was in the middle of texting her and I guess out of nowhere he realized I was standing there. I saw him look over his shoulder (without turning his head) and he immediately starts saying his daughter was out and needed to use the restroom but I'm not fucking fool!! Even if she did say that, it not what I happened to see. And no you guys, I was not intentionally peeping. He's much taller than me and it's the way he was positioned when I walked up. Almost like to have his back turned to left side maybe assuming when I walked back up, that I'd walk to his left but I didn't. Anyway, my heart sank to my butt. Before I knew it my hands started shaking and my chest felt heavy. I didn't show it though. I calmly asked who was the woman in the picture. It could've been anybody. Benefit of the doubt here but my gut told me otherwise. He asked "what picture?" I wanted to walk away at that moment but held my composure. I stood next to him silently. A few minutes later, he did the most condescending thing (well it felt like it to me) he slapped his hand on my shoulder and told me that I needed to calm down. As if I was acting hysterical. Before I could say anything he walked away! Stood at a distance from me and was silent towards me for 2 hours before I told him to just drop me off at home and I'd see him later.

Lastly, the night before, he was up very late (which is not like him) the kids and I call him old man because he's usually the first one in bed and doesn't wake up for anything unless it's an emergency. I asked who was he texting with at 230 in the morning because his notification went off,that's what woke me up. He claimed no one and opened a game on his phone. He didn't know that I saw him open the game and lied saying he'd been playing for hours. Maybe so but I know his text notifications. Like what's up? That wasn't a big deal to me until I was finally going back to sleep and he placed his phone under his hip. His phone is always on the night stand. I didn't say anything but he kept waking me up because he kept moving around. At one point he had the blanket over his head and was texting!!!! I could tell because of the tapping and all but one corner didn't his head so I saw it too but all I could see was the keyboard. The next morning, his alarm went off and we'll sometimes turn off each other's alarms. I went to turn his off but he had it under his pillow. I brushed that all that off until what I saw yesterday. He kept taking calls and texts away from me. That's fine, it's weird to me because he never does that not saying he never can. Even after making it home, he wouldn't talk to me. Sat away from me.

All I did was ask who was the woman in the picture? His behavior just seems off!

I'm not usually one to snoop.

How can I prove he's cheating again?

Edit# Thanks ladies. The first time he cheated he tried to make it seem like it wasn't more than what it was. He has a way of making me second guess myself about his behavior. Especially with this situation. I guess I need it to prove to myself that he isn't worth my time. I know it sounds dumb. I am deeply wanting this to work and probably sound crazy while at it.