The most obvious sign. If raw sewage is backing up into your lowest drains (like basement showers), your tank or its inlet pipe is severely clogged.
When you pour grease down the kitchen sink, it cools and solidifies inside your pipes or at the top of the septic tank, forming a hard "scum layer" that eventually blocks the inlet pipe. Sludge Overload clogged septic tank
Some homeowners use monthly treatments to boost the enzymes that break down solids, though a healthy tank usually maintains its own bacteria. The most obvious sign