One of the ways is to find the facebook pages of big dropshippers and check the ads that they are running. It's an open information these days.
Another way is to go to facebook ads library and look there.
The third way is to go to AliExpress, go through the best selling items, then search for these items in facebook search bar. Some of those posts were created to sell those items.
How to identify the ad is winning - well, there is no way to be 100% sure it's winning, but if you see hundreds of thousands of views/engagements on the ads, the chances are it's a winning ad since people wouldn't be spending that much money on something that doesn't bring the results.