how do I testfor blocks? like if I put down stone near a command block with a comparator attached to it and it will activate a piston connected by the comparator on the command block? please answer.
The command you want is /testforblock. The syntax of it is:
testforblock <x> <y> <z> <TileName> [dataValue] [dataTag]
So to test for a stone block at X=10, Y=64, Z=10, the command would be:
testforblock 10 64 10 stone