The internet and social media make it super easy to find somebody to cheat on your significant other with...and they also make it super easy to get caught.
People who have been completely and utterly screwed over by a girlfriend or boyfriend shared their stories about how they found out they were being cheated on. The incidents range from finding a smoking gun on Instagram to having their sister-in-law sneak them into a conference call.
The stories are as fun to read as they are horrible to experience personally.
Four years ago I was dating a girl who became distant and disinterested in a lot of things we both enjoyed together. Despite attempts to talk about it, she only grew more distant then became less available to hang out and suddenly was working late a lot. I started getting suspicious about cheating when she was on her phone nonstop. Again I tried to talk about it but she wouldn't engage. She also just didn't want to break up with me for whatever reason.
I was actually going to end it on my own, but one weekend when she was seemingly unavailable, some mutual friends told me they spotted her with a dude from her work and they were all over each other. Coincidentally, she had also lent me her iPad and it was synced to her phone so I saw lots of texts between those two over that weekend. They were talking about running away together, etc.