Describe where it returns
Say whether the toilet level rises or water enters a shower or basin. Include whether another fixture was used at the same time.
Keep use limited
Do not repeatedly fill the system to measure the return. A professional can inspect the connected route and model arrangement while considering contamination and access.
A useful distinction
Returning water immediately after stopping differs from a later rise following another fixture's use. That interval helps describe the connected system without relying on an unverified non-return-valve diagnosis.