Look at the pattern
Condensation often affects many nails, rafters and areas of felt at once. A roof leak tends to follow a more local route.
Check extract ducts
A bathroom duct ending or leaking in the loft can add a lot of moisture.
Recheck in dry weather
If the same wetness appears without rain, condensation moves higher on the list.
Want the roof looked at?
Describe what you have actually seen and the weather in which it happens. That gives the roofer a useful starting point.
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