How can I identify the cause of ceiling water damage?
Not sure if you removed the insulation to take the pics, but if not then missing insulation will create a cold spot which will certainly exacerbate condensation. 

I always start with a thorough exterior inspection if possible (to rule it out), but condensation is extremely common in older homes, especially where a ceiling meets an exterior wall. Poor insulation installation at this point is common (it's difficult for some guys to get it right down to meet the insulation in the wall or up to the wall plate etc. as it's often a small space). 

Just be aware that there may be an intentional air gap which the insulation should not block. Venting cold roofs is important to prevent warm air escaping the house and condensing on the underneath of cold roof coverings.