Measuring wattage to PoE device - how to hook up a multimeter inline?
If you don't want to measure at the source or at the sink, your only choice is to break the cable. Cut both +ve wires. Connect the two ends from the source together; connect the two ends to the sink together; put your meter between. You will have to break both wires to get an accurate measurement, but you must also allow current flow through both wires.