You do not actually always know if the supplier will be profitable at a tradeshow. You just communicate with the supplier, you make sure they work with Amazon sellers and sell brand name products, and then you get their contact info. Suppliers like working with people they meet at tradeshows as they see them face to face, and the sellers are more serious when they are attending tradeshows.
You can also bring a laptop with you, see if the supplier can send you a spreadsheet of products, and then you can run the spreadsheet through an analyzer tool. I have seen people take this route as well.
In the end, you are just trying to introduce yourself and get the contact info of suppliers, so you have people to reach out to. It is a good way to build relationships with suppliers early on.
What is the point in going to trade shows if we don't know the suppliers there are profitable?