You cannot use the ~ character in target selectors, meaning you'd need to rely on command syntax and not selector syntax. You cannot use relative coordinates with /testfor as its command syntax does not support it.
You would instead need to place an entity at the desired position and execute from it relatively using /execute:
/execute <target> X Y Z <command><target> is the target entity to change the coordinate origin to, while XYZ is going to further modify the origin. Using relative coordinates would cause "command" to be executed relative from the target's position.
Sample armor stand to create:
/summon ArmorStand X Y Z {Tags:["anchor"]}For example, assuming there is an armor stand tagged anchor at the intended location, the following tests for players relative to the armor stand:
/execute @e[type=ArmorStand,tag=anchor] ~1 ~4 ~-3 /testfor @a[dx=0,dz=0,dy=102]/execute @e[type=ArmorStand,tag=anchor] ~1 ~4 ~3 /testfor @a[dx=0,dz=0,dy=102]