How to find out whether potential advisor will retire soon?
By asking, "are you accepting PhD students in the coming academic year?" you will find out not only whether they are openly planning on retiring soon, but also critically whether their docket of students is full.  You can do this via email without risking offending them.  In your email, also justify why you are interested in working with them.

I personally have experience with this, and unfortunately the person I contacted was retiring in the next year, and they told me so when I emailed them.