Question on etiquette while trying to secure a postdoctoral position
How close are you to graduating? If you're still a few years away, then perhaps get an introduction from your advisor to that professor and see if there are natural ways to seek feedback or advice from that professor. 

If you are thinking about the next year and a half, then your questions should be more concrete. In particular, you might tie your question to funding, as that determines a lot of the possibilities. Even if a professor loves your work, they can't hire you if they don't have the money for it.


If you see from the professor's CV that there is an active grant, you could ask about whether there is space on that grant.
If there are regularly postdoctoral openings in the person's department, you can ask about whether the professor is looking for another postdoc soon. (You may learn that people in the department tend to take turns hiring postdocs, and the professor is (un)likely to be up this year. Apply anyway, but know your odds.)
You could say that you're preparing to apply for an external grant for a postdoc (e.g. NSF in the U.S.) and wondering whether the professor would be open to having you work with them on that. (Often a faculty sponsor is already required to apply.)