Recurring blockages are the single strongest indicator. A one-off blockage is usually something that went down the drain that shouldn't have. A blockage that returns every few weeks in the same place almost always means debris is catching on something structural — a broken edge, an open joint, a step where the pipe has dropped.
Above ground, look for a depression or dip following the line of the drain, damp or boggy ground with no other explanation, and a strip of grass that is greener and grows faster than everything around it. Rats need only a 40mm gap to enter a broken drain, so rat sightings near a manhole or in a garden are a serious signal.
None of these signs identify the defect precisely — they justify a survey. An HD CCTV camera pushed through the run will show exactly where the break is, how bad it is and whether it can be lined rather than dug up.