Look at how widely the moisture is spread
Condensation can appear on many rafters, nails and areas of felt at once. A rain leak is usually more localised.
Check extractor ducts
A bathroom or kitchen duct that ends in the loft can release large amounts of moisture into the roof space.
Look at the weather record
If the loft is wet after a cold dry night but stays similar during rain, improving ventilation and reducing moisture may matter more than repairing the covering.