When you look at the coil, the power side will usually have two wires going back to the playfield, the ground will be a single wire (there may be other wires that jumper off through EOS switches at the coil....talking about wires that leave and go elsewhere). Most times the ground wires are two color (green, yellow stripe for example) and the power wire is solid. The ground wire will go back to the MPU. The power wire will jumper to the next solenoid, hence why it has two wires on it (one in, one out). Also, look at another coil for reference. Lastly, you can take a meter, and putting one lead to the ground strap in the game cabinet, the other lead on the coil lugs, one lug will read ~50VDC (that's the power), the other one shouldn't read much of anything (that's the ground side, currently floating as the MPU isn't sending ground to turn on the coil).