What is the popular way of opening a conversation (call or email) with someone you haven't seen in a while?
1) Hi there, I was just checking in with you to see how you were doing?
2) Hi there, I was just seeing how you were doing?
more common ones?
I would say that in the UK "Just checking in with someone" is not very common. It is possibly more common in American, I don't like the expression, it implies there is a timetable of check-ins to follow.
I would prefer:
Just seeing how you are.
just checking you are alright.
just wondering how you are doing.
just calling to see if you are alright.