People persistently disagreeing with me is one sign.

The more numerous they are, the smarter they are, and the more coherent-sounding their arguments, the more likely it is that there's a problem at my end.

Of course, I often ignore such signals. For example, I still think that global warming is good, that our distant ancestors were probably clay minerals, that memetics is cool, that there will be a memetic takeover - and lots of other strange things.