One thing that seems relevant here as a distinguishing factor which helps point out some but by no means all "conspiracy theories" is that often conspiracy theories as such are long term and overarching. So for example, the Bavarian Illuminati were founded in the 1700s. Thus, a conspiracy about the Illuminati will claim that they have been running things behind the scenes for a long time. That drastically reduces their chances. Moreover, while there are many such conspiracy theories, they often label the group behind the curtain differently. Heuristically, conspiracy theories satisfying such properties should be assigned a very low value.

This doesn't really help though for quite a few conspiracy theories that are commonly ridiculed (e.g. Apollo hoax claims, and 9/11 Truther claims).