This is the easiest way to find your drain under your basement concrete floor slab. This is if you have a plastic drain pipe (like PVC or ABS).

I made an impromptu cellphone video of the process since I could not find anything online explaining the process:

You need:
An MScope
Fish Tape (metal)
Access to drain cleanout

And the rest is easy.

Products in this video:

Ridgid SR20 Line Locator:
http://www.amazon.com/gp/product/B001...

Ridgid ST305 Line Transmitter (should come with the scope):
http://www.amazon.com/gp/product/B001...

Greenlee Steel Fish Tape:
http://www.amazon.com/gp/product/B000...