Not being able to tell ahead of time what materials are present in the dungeon makes sense imho, your party shouldn't really have any fore knowledge of that.
You get two options for gathering the materials, half now for a small stamina cost, or leave until later. If you leave till later you can gather all of those materials in the same way you did in the previous versions.
There's a show/hide local resources button near the top right of the dungeon screen, that will open window near the bottom that shows the resource types/amounts and gives you the assign workers button, you can also just assign your workers from the Mansion screen.