Today septic tank risers are often installed two inches above grade. Risers, lids, and/or manhole covers are generally easy to spot. However, older tanks might not have visible parts.
Soil absorption systems may be depicted by shallow, parallel depressions, marking the drain lines. It is uncommon for large trees or rocks to surround the drain field.
The “as-built” drawing, if one exists, maps the septic system installed for your property. Contact the county sanitarian department to request records (often requires payment).