Finding the direction of plumbing under the basement floor?

I'm considering building a basement bathroom, but there are no rough-ins. I would have to cut open the floor to access the plumbing, but what would be the best process to follow in finding an access point? Any suggestions, and/or websites for this kind of stuff would be greatly appreciated. Yes, the plumbing connects to the city sewer via the main sewer line that runs under the house. And I understand the process involved with venting and so on. Can someone tell me how to determine which way the sewer line runs under the house?