Keep the baseline
Ask for available appliance paperwork and describe normal clearing after ordinary use. Do not test unfamiliar fixtures repeatedly to map the pipework.
When to request assessment
New leakage, water returning or prolonged running deserves a precise enquiry. An engineer can assess existing equipment where suitable; the service is not a new installation or a redesign of the property's drainage.
A useful distinction
Previous occupants' descriptions can be useful history but should be kept separate from current observations. The installed model and present clearing result provide a firmer basis for deciding whether an assessment is needed.