Time the fixture and watch the pattern instead of naming the obstruction.
The short answer
Note how quickly water begins to pool, whether the drain eventually clears, and whether the problem changes with flow. Check for normal drainage at other fixtures. Stop if testing would overflow. A local stopper, trap, branch, or vent condition may be involved, but the visible symptom does not select a method.
What to notice
Separate fixture behavior from line behavior
A bathroom sink that drains slowly only with the stopper engaged presents different information from a shower that remains slow after surface debris is removed. Record gurgling, odor, and water movement at nearby fixtures. Do not dismantle a trap if you cannot control remaining water or reconnect it safely.
Tell the provider what has been tried
List plunging, mechanical tools, cleaners, or previous service and when each occurred. Never mix drain products. A chemical in the line changes the provider’s safety preparation. A temporary improvement is useful history, especially if the drain becomes slow again after the same kind of use.
Define success beyond fast flow
Ask what caused the restriction, what section was addressed, and whether the result indicates another issue. A flow test should not create damage. If a recurring condition suggests an inspection or piping repair, request the evidence and alternatives. Keep the invoice specific to the fixture and method used.
One useful question
Does a slow drain mean the vent is blocked?
A vent issue is one possible explanation among several, not a conclusion from slow flow alone. Gurgling and interaction with other fixtures can add useful observations. A provider needs to assess the actual configuration and line before recommending vent work.