Mark, Are you trying to find studs to drill on them or are you trying to avoid studs/ Either ways a stud finder should help locate studs for you. Remember if you are drilling to hang something heavy you better drill into the metal stud, use a self tapping screw designed for metal studs.

Metal stud finder usually available at your local hardware store.

Mark, just to clarify your question -- even if you know where the studs were, you're wondering how to know how to avoid the plumbing running in between them? To plant sinkers, I've always tried to drill very gently through the drywall, just enough to get to the other side -- most drywall is a half inch, so you can measure it on your drill bit and mark with masking tape. Then you can feed a thin screwdriver through to make sure you aren't going to hit anything. If you have plastic pipes, ...»