If you are asking how can you tell if it is the inner or outer that has gone bad.
First you have to jack the car up and support it. Then while looking behind the tire, push and pull on the tire on the side closest to the tie rod end. While you are pushing and pulling you will be able to see if it is the outer as the pivot joint will move around inside the steering knuckle. If you don't see any movement in the outer. Continue to push and pull on the tire with one hand and at the same time put your other hand on the inner tie rod, If you feel it jerking inside the boot...it's the inner tie rod joint.
Play when you push left/right (side to side) will usually point you to the tie rod. Now you just have to get in behind the tire as you are pushing it to see exactly what part is moving for sure. No need to replace both if you don't have to.