There is testing available.

There is nothing wrong with bacteria, we could not survive without bacteria. Some bacteria, as well as viruses are deadly to people.

Ever noticed how well lawns or other plants grow over a septic drain field? Whatever is in there creates a very biologically healthy soil.

It's nothing I would worry about but if you have concerns, do get it tested.

Where do you get soil tested? and what do you test for?

You get soil tested by soil testing labs :-) Google that with your state and you will get a listing of local labs. Some states' extension service still do soil tests but they are becoming less available and often tend to be pretty simplified and not what you need.

I would test for any heavy metals at least. These can sometimes be more critical and of a health concern than other factors. You can also test for pathogens but you will need to be specific about what pathogens you want the soil tested for. Since many bacterial organisms have natural predators within the soil biology or are degraded with exposure to sun, air and time, I would probably skip that.

Alternatively, I believe every state (sometimes even down to the county level) have septic system inspectors or inspection offices and they may do testing on failed septic fields as well.

I would work it into soil for planting, but to be on the safe side would not use for a vegetable garden. There are human pathogens that might survive in the septic tank.

Start with your local septic service company. Explain your concerns and ask for references.