If the mountain is big and high enough to have Obsidian, it can also have silver veins. We never found a single silver vein in an area without Obsidian. If you find Drakes, Stone Golems and Obsidian, you're in the right area.

No need for wishbones if you have Iron Sledge and/ or Stagbreaker. Just slam the ground and if you get a "Too hard" message with no Obsidian around (so mine this first!) you found silver.