Another thing you can do is disconnect the wires and measure the DC resistance. A 4 ohm driver will usually have a DCR of around 3 ohms, an 8 ohm driver around 6 ohms, and a 16 ohm driver around 12. These are just ballpark numbers, but you should get the idea. If you know what the impedance of the entire cabinet is, you can measure the cabinet's DCR and use that as a reference. If the cabinet's DCR is 3.2 ohms, and an individual driver's is 12.8, rest assured you're dealing with 16 ohm drivers.