I think that it is relatively easy to pin forehand lameness on the shoulder if there is no other symptom aside from the lameness itself. If there's no obvious injury, no heat, no swelling, etc., the shoulder is convenient. There's a lot of "stuff" in there to be sore, muscle, bone, tendon, that are not immedately visible and can be assumed to be a hidden cause.
It is certainly very important to correctly locate lameness, and a vet is probably the best (excepting perhaps the rare old timer with years of experience, common sense, and a good eye) to say where the pain is actually coming from.
My personal philosophy on lame horses is to try to find a reason. Find the injury, heat, swelling, whatever. If I can't find it, I'll assume the horse slept wrong (shoot, I do that and wake up stiff), and keep an eye on him. If he isn't better by the next morning, I check the horse again and think about calling a vet or other pro if I still can't find the cause. To blame lameness on anything you can't directly observe without being a vet or extremely experienced person with lameness seems like taking quite a chance, IMHO.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown