how to measure the current from AC-DC supplier?
The reason you're getting AC voltage is that you're pulling more than the rated current, so your output has a ripple in it. If you're drawing too much current, the output filter capacitors will be incapable of "smoothing" the AC (from either the switch-mode transformer, or the linear transformer -- whatever the PSU is using). So, although you may be getting 12V, it's pretty nasty 12V.

I bet if you increased your load's resistance a bit (therefore decreasing current consumption), you'd see the AC voltage drop to near-zero. Otherwise, it's not much of a DC power supply, now is it!