That's the million dollar question!
There is definitely a few different resources and tools you can use to see what products are popular for dropshipping. I use these all when scouting for products for a new store I am building.
First I start with the Explore Page and use the sort feature on the right to sort by orders. This way I can see exactly what customers are buying the most of!
Then I jump to Aliexpress and start by going through their Bestsellers List as it is updated weekly and you can sort after by category you are selling in.
If you've got a product in mind I would do an Aliexpress search to look at order counts and reviews. As a standard I suggest picking products with a few hundred orders so you know it's popular with customers!
Otherwise it doesn't hurt to check out Google Trends to see what's going on there too with the niche or product idea. I wrote an article recently about top selling products which I think you may find useful here as well! Hope this helps!