Slow and backed up drains may or may not be a sign of a full septic tank. First of all, make sure it is not simply a clogged drain. Perhaps the easiest way to determine this is by checking other plumbing fixtures in your home. If there is only one fixture draining slowly or not draining, you can be fairly certain you are dealing with a clogged drain. On the other hand, if all drains are slow or waste is backing up into the lowest plumbing fixture in your home, suspect the septic system.