Apart from the obvious way to contact every product's seller one-by-one, here's a trick that helped me a lot spotting them fast:
In the search bar, write a category you want to find items from, along with the word "dropship" or "dropshipping", "drop shipping" etc.
You'll be surprised how many sellers have put this word in their title. This method provides a bonus that these sellers are probably more familiar and experienced with the dropshipping method.
Nevertheless we must always check seller's feedback score, if the account is aged, etc