They Shower As Soon As They Get Home
You might not notice this one at first because it does not involve keeping secrets and hanging out with new friends. If it happens for a period of time, then you will notice it. You know that your partner took a shower in the morning before going to work but now they are starting to jump in the shower as soon as they get home.
They do this for a pretty obvious reason and that is to remove the smell from the person they had an affair with. There are other ways to discover this.
You can volunteer to do the laundry for the whole week and start going through their clothes and try to find a scent that you’re not familiar with, most probably a perfume from the opposite sex.