They Change Their Routine
You notice how they start doing different things, maybe they have a new hobby or they started taking dance lessons and you know that they have never before expressed a desire to do that before. They are up and running in the morning and you also notice unexpected euphoria in their mood. If the changes are gradual that might not be as suspicious than if they changed overnight.
A smart thing to do is to check their schedule and if there are major changes then ask questions and try to find out why those changes happened in the first place. If there is no particular reason for them then guess what? They might be cheating on you behind your back. It’s always smart to keep track of their habits and routines.