Your CDI has two plugs, a 4-pin and a 2-pin. The 4-pin side may have 3 or 4 wires connected to it (both AC and DC versions). If the 2-pin connector has only 1 wire going to it, it's a DC CDI. If there are 2 wires attached to the 2-pin connector, then it's an AC CDI.
Note: A very few DC-powered CDI systems will have two wires on the two-pin connector. For these systems, please check to see if your headlights will work while the engine is not running, and if your stock CDI has "DC" printed anywhere on it. In these cases, you need a DC CDI.