Minecraft TestFor command help
Solution 1:
Your command does not work as supplying a x, y and z modifier only changes the location of which the command block detects from.
If you'd like to detect for a player at that location, add the r (radius) modifier to your selector.
By default, the radius (if one is not specified) is infinite (also called 'none').
You'll want to use:
/testfor @a[x=-436,y=9,z=-758,r=1]