Rear Suspension Ball Joint

Lift up the vehicle until rear wheels are off ground.
Grasp the bottom of tire and move it in and out in axial direction.
If movement is observed between the brake disc cover (A) and end of front lateral link (B), ball joint may be excessively worn.

Fig 1: Identifying Movement Between Brake Disc Cover And End Of Front Lateral Link
Courtesy of SUBARU OF AMERICA, INC.
Grasp the end of front lateral link (B) and move it up and down. If movement is observed between the housing (C) and front lateral link (B) boss, ball joint may be excessively worn.

Fig 2: Identifying Movement Between Housing And Front Lateral Link
Courtesy of SUBARU OF AMERICA, INC.
If the movement related to the previous two steps is observed, replace the front lateral link. Refer to FRONT LATERAL LINK RS-22, Front Lateral Link.
Damage of dust boot
Visually inspect the ball joint dust boot. Replace if front lateral link is damaged.
Check upper arm ball joint in the same manner.

Fig 3: Identifying Upper Arm Ball Joint
Courtesy of SUBARU OF AMERICA, INC.