1) Give them a realistic task to work on - this is what you want them to do if you hire them 2) Give them a reasonable amount of time 3) Pay them for it - why wouldn't you?
We did this for one candidate and although your gut feel as an employer is sometimes, "is this a waste of £1000"? but if you are already quite far on in the process, you probably think they might be good enough and that £1000 is offset against the Recruiter fee that you incur when a hire doesn't work out or the loss of a good person you don't hire because you are not sure.
It also shows good will between employer and applicant. We have done it once so far and didn't take on the person but gave them feedback on the task and because they were paid, they didn't see it as a waste of time.