Subluxations usually pop back into place on their own. Often, dislocations need to be put back into place by someone else. Because there is such a wide range of instability, it is possible to have a shoulder that is unstable and not realize that the the shoulder is coming out of joint.
Symptoms usually consist of pain, especially when the arm is put into various positions.
Instability is usually classified by the direction the head pops out of the socket. Inferior instability is usually combined with anterior instability - the head falls out below the socket. Multidirectional instability means that the head is very loose and can subluxate out the socket in more than one direction. Generally speaking, people with traumatic instability experience dislocations while people with atraumatic instability experience subluxations. I dislocated my shoulder skiing.