We have a wall between our dining room and living room (there's a doorway at the far end). Not sure if it is load bearing. I tried to use a stud sensor to see if there was a header - not sure how accurate. If it is load bearing - is there anyway to cut an opening in the wall to open up the 2 rooms more?

 

If it is load bearing, you need to size your header for opening width and what it is holding up.

If you don't have floor plans, you need to look either above or below the wall of interest. If it is load bearing, there will be something (like rafters or floor trusses) sitting on top of it and there will have to be something supporting the wall under it (like another wall and a foundation).

Decide how big of an opening you want, and then go to a lumber yard and they will tell you how big of a header you need.

Example:
I just bumped out a living room exterior wall. 24 foot removal of a bearing wall. The wall was holding up one side of 28 foot roof trusses. The solution was to use 3 2x14" microlams and hang the roof trusses so the ceiling was continuous.