I'm not sure what you mean by check valve. Do you have some sort of backflow prevention device to keep sanitary/storm water out of your house from the city pipes? If something is in the way it can be relocated or boxed in, perhaps in a fake closet. As for where they go under the slab, a camera can be inserted and then a special device used to locate it under the slab. Any company that does sewer repairs or replacement should be able to help you out.
You should be using pressure treated lumber when the wood (eg bottom plates) comes in contact with concrete.