Thats a pretty vague Question..........but in general lets say you have an outlet....A receptacle or light fixture..you need to remove the outlet from the wall, or the light.... In the junction box you usually have  2 sets of wires. 1 set is the IN and the other is the OUT....With a tester from one white and one black coming in from the same connecter put a tester across the two leads the IN will give you a reading the OUT will be dead or have no power