I most likely lack gaydar. I am nearly always surprised when somebody comes out. I have had friends tell me that it is 'obvious'... Seriously the only ones I suspect of being gay have to be flaming obvious. Then even sometimes still I don't 'get' it.
But that may have much more to do with how I see the world in a sexual context.
For instance this image:
I see the dolphins first, then I see the other with a little squinting.
And this image:
I took someone else pointing out the fact that they are not really dancers....
I most likely do not view people in context of sexuality, thus I fail to see 'gay' or 'straight'.
This may explain why I have only had 8 sexual partners in 45 years of life too.... but that is a different story.