If it rained quite a bit just prior to the water stain appearing, then I would concentrate my efforts on the exterior of the house. But if it did not rain, then please consider the following:
If the stain dries out and doesn't come back, I wouldn't worry about it--probably just an overflowed toilet or shower curtain that was not doing its job. But if the ceiling stays wet, I'd definitely poke a hole in it and let any water drain out that may have accumulated. Then I'd have someone go upstairs and flush the toilet a few times, then wait a few minutes and run the water in the bathtub, sink, etc.. I'd also check the plumbing behind the shower access panel. Also, check to see if any tiles are loose in your bathtub/shower, or if any grout or caulk is missing. If it is missing grout or caulk, pour water on that area to see if it is the source of the leak. Also, just because it is dry below your p-trap in the sink doesn't rule out the sink drain as a culprit. See if the tailpipe is loose that screws into the fitting against the wall. The drain could be leaking behind the cabinet or in the wall.
If there's a large leak, either in one of the drain pipes, valves, or supply lines, it will be easy to troubleshoot. For small leaks, this method may not work, since the drywall can soak up a lot of water before it becomes saturated and lets the water flow out. Also, leaks in copper pipes that are in the wall or ceiling are difficult to troubleshoot. In these cases, you may have to cut out a large section of drywall--but that should be your very last resort, since you'll obviously have to repair all the drywall you cut out.
Eric