check up on = find out what somebody is doing and if he/she is telling the truth
examples:
- Could you please check up on the baby? I think I heard her cry.
- I 'll check up on the kids in the other room.
- The police checked up on him. On the day of the murder he was abroad.
- I 'm feeling fine now, mum. You don't have to check up on me every two minutes!
- How do you know where I was yesterday? Have you been checking up on me?
- I 'm going to check up on his story. I have the feeling he is lying.
- The new boss is checking up on us all the time. He is very annoying!
