Depends how much you want to spend. Volatile organic compounds, TCE, lead, RCRA 8 metals, hardness, pH all kinds of things to test for. Where do you live?

There are a few water testing labs out there. But types of tests you want will increase the costs.

There is a lab, www.EMSL.com. You can go there and get the jars and have them run every test under the sun for your water.

My house has a 300’ well, it had a softener when I bought it, and I always had slimy water. I put in a whole house filter then had rain soft put on a carbon filter and Remove the water softener and had great water ever since

IMHO paper filter then carbon filter will clean out just about everything. They come every 5 years and replace the carbon, about $1,500 to install and $800 to recharge.