Lying down may make this more easier.
Place your hand on your abdominals just below the rib cage and inhale and exhale like normal.
When you exhale, how do your abdominals feel- do they feel they aren't really doing anything or can you feel them ever so slightly contract as you exhale?
Reason I'm asking is I have a diastasis recti starting at the sternum and my abdominals here are very weak and I suspect it's affecting my breathing which is most definitely not correct. And I've forgotten what "normal" looks like.