Classic signs are damp on an internal wall that corresponds to the position of an external gully or drain run, tide marks that do not respond to a new damp-proof course, and damp that is worse after heavy use of water in the property.
Damp specialists frequently treat the symptom for years without anyone looking at the drainage. A CCTV survey is cheap by comparison and rules the cause in or out definitively.
Long-term leakage also washes fines out of the soil beneath foundations, which is a subsidence risk, so it is worth resolving quickly.