You will find it advantageous to scout out all the mountains in the area surrounding your mountain. You will likely find that some resources are not available, or are expensive, in the Regional Market. You can later send out mining crews to collect resources from these surrounding areas, although slowly. If a player later occupies that dungeon, you will still be able to see what resources were available in that dungeon. There isn't much of a need to scout farther than you would be willing to send goblin mining crews or raiding parties. Two to three overworld map tiles aren't bad. Beyond that can take some substantial time. It's rough to for a few goblins to spend 4 days traveling and a day or two of mining, just to get what you could have purchased or captured from a settlement.
To survey a dungeon, go to the overworld screen, pick a mountain and click the magnifying glass icon (View) on the bottom right to view it. Then click on one of the square or circular icons (dungeons) within the mountain. Once you have clicked on one, new icon(s) will appear on the upper left corner of the screen. Look for the one that is for Surveying (triangle with an eye in it), then click that.
Your Goblins will now go forth and scout the empty dungeon. After surveying, you can tell what resource nodes are available in that dungeon, and the options to mine or occupy it become available.