How can I locate the water main entry point to my house?
Is there a water shut off at the street? Should be a small manhole, maybe 6" wide. You should be able to follow that in a straight line to your house. From that point, inside or outside of your house, you should be able to find a copper pipe and a ball valve. Maybe the copper surfaces above the slab for an indoor shutoff, and you could attach the filter there.