This isn't contractual, but I always ask "so, how many days did you take last year" or "how many days does the leadership team take" or "how many days does the team take on average".
Give me a feel for what I'm walking into.
I don't know if a company with unlimited PTO would put it in a contract, because that might put them on the hook for a payout for unused vacation when you leave. IANAL.