How to use /testfor to sense mobs?

While DominatorX's solution is perfectly valid - the command you are looking for might look something like this:

/testfor @e[type=Mobtype*,x=x,y=y,z=z,r=r**]

*Mobtype would be replaced with the mob you are looking for, like Skeleton will search for skellies, and Zombie will search for zombies. Alternatively, if you don't need to specify the type of mob, you can remove the [type=Mobtype] all together. This will just search for any entity.

**this can be used to specify a specific location, and a radius around that location. Alternatively, you can just use r=r and it will test within the point of origin (the command block)

USE PRESSURE PLATES!

You're really overthinking the problem.