How do you tell if ignition coil is 6v or 12v?

To determine if an ignition coil is 6v or 12v, check for any labeling or markings on the coil itself, which often indicate the voltage rating. You can also measure the resistance on the primary winding using a multimeter; typically, a 6v coil has lower resistance (about 1-3 ohms) compared to a 12v coil (about 3-6 ohms). Additionally, the coil's application can be a clue, as older vehicles often use 6v systems, while most modern vehicles operate on 12v.