On that note keep in mind that most splitter ports are a dead short to DC. Also a staple through a cable is going to be a short also.
The OP asked about an easy way to identify the feed when cutting a cable to insert a splitter. Many times it is difficult to determine where the other end of a cable is so it is not always possible to disconnect the far end to put a tester on it or even know what it is connected to. The only way to handle this is to look for the end with signal on it then you know it's the feed. Only way to do that is with a signal level meter or a TV. There is no cheap tester that will tell you if you have signal.
-Hal
