Grab the cylinder and push it back and forth lengthwise. If it moves forward and back, that's endshake. A tiny bit is ok, a lot is not.
2 types of endshake: cylinder/yoke endshake, and yoke/frame endshake. The cylinder can move too much on the yoke barrel (the center axle of the cylinder), or the yoke can move back and forth in the frame. Both types are repairable unless someone has just seriously screwed stuff up. ;-)