That depends on system design. Older systems tend to be combined systems, unless they've been reworked to separate stormwater. If you live in or near somewhere

That depends on system design. Older systems tend to be combined systems, unless they've been reworked to separate stormwater. If you live in or near somewhere that often has sewage outflows in heavy rains, chances are it's a city that urbanized before WW2 and has combined sewers. Sanitary sewers also tend to be leaky, rain will result in surface water entering the sanitary sewers and larger flows into the treatment plants even if storm drains are routed elsewhere. You might get some filtration from soil though. If your city only very occasionally has sewage outflows in heavy rains, it's probably from ground inflows rather than combined sewers and there may also need to be a second trigger such as power loss.