1.8.9 Tesfor specific player at a specific xyz
Solution 1:
Selector arguments can only be used with a selector (@p, @a, @r, or @e). There must also be no space between the selector and the selector arguments:
/testfor @a[x=506,y=4,z=-879,r=1]
If you want to include a name, you can use the name= selector argument:
/testfor @a[x=506,y=4,z=-879,r=1,name=mastermc123]