It depends upon where you can access the sewer line to insert the "test plug". You would then fill the piping from a point behind the plug to a certain level. Other plugs may be needed for toilet pipes, tubs, showers etc. anything with a low outlet.
But if you can't see the entire sewer line, it would be very difficult to find the leak if it happens to be in a spot that you can't see. Sometimes if filling the lines with water doesn't work, smoke is injected into the pipe lines and is more easily visible when it escapes.
However, a better method;
Floor replacements are very expensive, for that reason I would strongly recommend that you hire a plumbing company that uses cameras to find defective pipes and joints in sewer systems. The cost of that would be minimal to a future floor replacement.
Also, the camera should be able to see and tell you exactly where any breakage may be.
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