In my living room I have noticed a long (about 3-4 feet) crack or weakness in
the ceiling. It's not really a crack but there is definitely a weakness. It
seems to be near a joist. An upstairs bath is above this area of the living
room. Although I have never seen any leaks or water spots I wonder if there was
a previous leak of some kind that may have damaged the subfloor. Or is it just
a joist "settling" in this 20 year old house that we bought in 1999 ? What type
of service person could help me determine exactly what the problem is ? Thanks

What type of service person could help me determine exactly what the problem
is ?

Structural engineer. Might be pricey, but if you need to sleep at night it
would be worth it. The average "home inspector may not be anywhere near well
qualified enougfh for this type of work. Good luck.

Joe