They Start Hiding Their Phone
You see your partner texting more often than usual and sometimes when they go to the bathroom they go there with their phone. You occasionally notice a smile or two and when you ask them about it they always say it is nothing.
You also try to have a peek at their phone and they are evasive about that. This secrecy seems really suspicious to you and for a good reason. If you try to ask them about what they are smiling about or who they are talking to, they probably won’t want to tell you anything or the question itself always leads to a fight.
They tell you things like the fact that they are entitled to their privacy and you don’t have any right to intrude upon that. Be careful around people who manifest behavior like that.