Before you search for a plumber or drain specialist, spend two minutes testing whether your problem is isolated to one fixture or systemic. Denver homeowners who do this describe the situation accurately on the first call, which affects dispatch, pricing, and urgency. Denver Drain Cleaners uses that same intake on the phone.
The upstairs/downstairs test
Run water in a upstairs sink for two minutes while someone watches the lowest drain in the house, basement floor drain or first-floor tub. If that lower drain gurgles or rises, you are likely dealing with main line restriction, not a upstairs branch clog alone.
The toilet-tub relationship
Flush the toilet nearest the problem. If the tub or shower trap bubbles or water level moves, air in the system is fighting a blockage downstream of both fixtures, main line territory.
Single-fixture isolation
If every other fixture in the house behaves normally and only one kitchen sink is slow, you are probably in a branch line serving that sink. Plunger and hand auger are reasonable first steps.
When the test says main line
Stop running water. Do not flush repeatedly to confirm, you already confirmed. Call someone equipped for main laterals, not a fixture-only visit.
Denver Drain Cleaners published a longer version of this decision tree for local homeowners: clogged drain vs sewer line problem. The quick test here matches what their phone intake asks anyway.
Document results
Text yourself which fixtures reacted. That note helps the technician bring the right machine first trip, especially important on weekends when return trips are painful.