I tried 'they will have passed (ie, spent) a few minutes'. I don't see how to tell whether the noun phrase that follows the verb is the subject or a modifier - ie, I can't tell whether this phrase means 'a few minutes will have passed' or '(they) will have passed a few minutes'. Any thoughts?
well it's often indistinguishable, i have to admit, but not this time. here, "passare" has "essere" as its auxiliary verb (even if it's in a third-person plural future form, it's still an "essere" verb), it can't mean "to pass time". that one would take "avere" (avranno passati alcuni minuti) to have that meaning.
Thanks - I also came across http://italian.about.com/library/verb/blverb_passare.htm, which says that the transitive use of the verb uses avere while the intransitive uses essere (this is just another way of saying what you've already said). So, since saranno is used, this means that there's no direct object and the 'alcuni minuti' must be the subject - ie, 'a few minutes will have passed'.