Pay attention to the whole system
One slow fixture may be a local clog. Multiple slow drains, gurgling, sewage odors, or wastewater backing up can point to a larger issue.
Check the yard
Unusually wet soil, bright green grass over the drain field, or standing water in dry weather deserve professional review. Keep vehicles and heavy equipment away from the tank and drain field.
Keep records
Write down pumping dates, repairs, and the location of system components. Good records make future service faster and more accurate.