I've been dealing with septic a lot since we bought our place 5 years ago. I had to more than double the existing drain field to build my shop (long story) and I did all the permit drawings and submission, so I feel I've begrudgingly become a bit of an expert on septic in my area.
Your answer is simple. Call a local pumper and have them come out, pump the tank, and do an inspection. They (may) contact the authorities and get an as-built for the system (get a copy for your records), locate the system components, pump the sludge, and perform a drain field test. If you have a diverter valve, they should also show you where is it and how to operate it. The cost should be ~$300. Follow whatever pump schedule they suggest, generally 2-5 years. I'd increase that by a factor of how many women are in the house. {This still amazes me; I recall using about pack about every six month when I was single. I can't even keep track now, but the kids are never lacking to roll tubes for crafts...but I digress}
If your drainfield uses perf pipe and gravel, I'd also recommend installing a tank filter to keep solids out of the field. My addition used vaults, so solids were less of a concern. My existing uses Orangeburg pipe, so it's FUBAR anyway.
Regarding pumping schedules: your mileage may vary. My mother just had her septic pumped for the first time. The house was built in '77 and had four of us there the majority of that time. The system was functioning fine- the city put in sewer and required she attach and decommission the septic system.