Relative humidity (RH) is the single most useful number for diagnosing a condensation problem. It measures how much water vapour the air is carrying relative to the maximum it could carry at its current temperature. Because warm air can hold far more moisture than cold air, the same amount of water vapour gives a low RH in a warm room and a high RH against a cold wall.
Indoor RH between roughly 40% and 60% is comfortable and low-risk. Sustained RH above about 70%, especially overnight and in bedrooms, indicates that moisture is being produced faster than the building removes it — a ventilation, heating or insulation issue, not a tenant behaviour issue. A cheap hygrometer left in the worst-affected room, read first thing in the morning, will show the pattern.
As humid air touches a surface colder than its dew point, its local RH hits 100% and water condenses out — which is why mould concentrates on cold spots.