Faint discolouration on ceilings
A pale yellow or brown tinge on a ceiling, even without an obvious wet patch, is often the earliest visible sign that water is finding its way in somewhere above.
A musty smell in the loft
Damp timber and insulation give off a distinctive musty smell well before any visible staining appears. It's worth a quick check in the loft periodically, not just when something looks wrong.
Small amounts of debris in the loft
Fine grit or granules from tiles collecting in the loft space can indicate the roof covering is starting to deteriorate, even before it actively leaks.
Peeling paint or bubbling wallpaper near the ceiling line
This is often an early sign of moisture getting into the wall cavity from above, well before it shows as a stain.
If you notice any of these, it's worth getting a proper leak survey before it becomes a bigger problem.