From Technic Wiki
A Spyglass is an excellent tool for identifying coordinates of blocks. Right-clicking with the Spyglass in hand will identify the block under the crosshairs, its item ID and its coordinates. It is not affected by water, lava or fog - if the block is in a loaded chunk and seen by the player it can be identified.
The Spyglass will not identify mobs and non-block entities such as fire, portals (though it will identify the obsidian portal frame), minecarts and rocket ships.
Holding Shift will allow you to identify accessible blocks (chests etc) if they are otherwise within reach.
The Spyglass is useful in the Nether where it can be used to identify the Nether Brick of fortresses through the darkness and fog, and for identifying ores that are in difficult to reach places to determine if it's worth the effort to mine them.
If the block name is unknown due to mod compatibility issues, the ID and coordinates will still be displayed.