How to find a key in a property list?

[Kalle Olavi Niemitalo]

Given a symbol SYM and a property name PROPNAME, how do I check
whether the symbol has that property?  (get SYM PROPNAME) is no
good because the property might exist with a value of nil.  (get*
SYM PROPNAME 'some-other-default) doesn't work either because the
value might happen to be some-other-default.  I came up with
this:

  (let* ((default-magic (list nil))
         (value (get* sym propname default-magic)))
    (eq value default-magic))

which is rather ugly but does the job.  It also conses on each
invocation; if I avoid that by using a quoted constant, is it
possible that the compiler coalesces the constant with some other
constant and I get false positives?  Which is more expensive, an
one-element list or an empty vector?  Would an uninterned symbol
work better?

Writing this, I suddenly realized I could call get* with two
different defaults and compare the results.  But this feels like
duplicating work.

Should I use an explicit loop?  Can that be written legibly with
`loop'?

FYI, the package I'm writing implements documentation strings for
names of symbol, text and overlay properties.