How do I write a command that tests for shears? [duplicate]

Simple, just use the updated /clear command. Here's an example of a setup to open an iron door:

#1 /setblock <x> <y> <z> redstone_block 0 destroy
#2 /clear @a[r=10] <item id> 0 0 {display:{Name:"<custom item name>"}}

Here's the setup of the command blocks:

[] = iron door
{} = comparator
() = redstone_block
#1 = command block 1
#2 = command block 2

#1()#2{}[]

To explain exactly how this works, I'll go command by command. The first command block sets up a constant redstone loop, which will be constantly activating command block two. Command block two is testfor the item in the player's hand if the player is within a radius of ten blocks. Then command block two outputs a redstone signal which activates the iron door. Also, here are a few commands you might want to run in order to not lag your world:

/gamerule commandBlockOutput false
/gamerule doTileDrops false

Lastly you must play in the latest snapshot of minecraft for this to correctly work. I hope this helps!