Count fixtures and levels
One slow sink can be local to its trap or branch. Several fixtures changing together—especially at the lowest drain—can point to the building drain or private lateral.
Run no extra water just to test a suspected main blockage. Added flow can worsen a backup.
Use timing as evidence
Note whether the symptom appears when a toilet flushes, a washer drains, or rainfall is heavy. Dry-weather recurrence and storm-only recurrence support different lines of inquiry.
A municipal surcharge event and a private restriction can overlap. Storm timing does not automatically place responsibility on the City or the property owner.
Know when the scope changes
Move from a fixture-cleaning conversation to a wider sewer assessment when several fixtures are involved, the lowest drain changes, or the blockage returns after clearing.
The next honest step may be access review, cleaning, camera inspection, or municipal guidance. It depends on the pattern and the condition found.