A simple toilet clog usually affects one fixture. Water rises in the bowl; other sinks and showers still drain normally.
A sewer backup often shows up as multiple fixtures draining slowly, water appearing in a tub when you flush, sewage odor, or a cleanout overflowing outside.
If you suspect a main-line backup, avoid running water elsewhere in the home. Stop using sinks, laundry, and other toilets until a plumber assesses the line.
Emergency partners can stabilize the situation, clear accessible blockages, and recommend camera inspection when roots, grease, or collapse are possible.