What does "I was just checking" mean?
In which situation, you would say:"I was just checking" for make sure something?

Question

Answer

I was just checking to make sure: in other words it means you were just verifying something .. making sure that if everything is ok .. its like you are controlling its state .. 
Example: I was just checking to make sure if I've got any message
               I was just controlling my email to confirm if I've got any message

hope that it helps : )