I find that I can often tell based on a baroque reading of the credits. Some of the 3D "upconversion" houses are better than others at their job. Some of the special effects houses have dedicated stereoscopic staff. Some of the studios themselves have dedicated stereoscopic departments. Obviously too, if there were stereoscopic cameras/film units involved in the main production crew of the film, that's usually a sign that the director cared about the stereoscopic output of the film.