How to avoid crossing my advisor if I write a paper as a sole author?
If you don't know whether your advisor expects to be a co-author or not, just ask him.

If you feel that the advisor shouldn't be a co-author, you can present that as the "default":


  I am planning to write up my work with X, to submit to conference Y. You haven't really been involved in this work so far, so I guess you won't co-author this paper with me. Is that right?


If based on the specifics of the situation you don't have any strong feelings against co-authoring this paper with him, but you just aren't sure what he wants, you can just ask him:


  I am planning to write up my work with X, to submit to conference Y. Are you interested in co-authoring this paper with me?


As you have said, you can't read minds. The only way to know what's in his, is to communicate. 

And it's really important that you have this discussion as soon as possible; authorship disputes are much trickier to resolve after publication (when they can potentially lead to retraction) than before submission.