Perhaps contacting their place of business anonymously and asking if the person is available. Or even asking a family member or mutual friend could get you clues on the person's status.
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If you have their full name you can look them up in the social security death database. It's morbid, but effective, though it may be a few months until the records are updated.

http://ssdi.rootsweb.ancestry.com/

I unfortunately found that a childhood friend had died this way. I had needed to confirm the information told to me by another friend because that friend was known to be a pathological liar.

You might also search for online death notices/obituaries in local papers.

If you don't know the person's real name, because they are primarily an internet contact, you've got less of a chance to find them. Your best bet there is to talk to mutual friends, or see if you can find them in other places they hang out online--an Internet forum or some such.

If they truly owe you something, it may be the better legal and professional route to call a collection agency. That way you are protected in case the other person files a stalking or harassment charge.

Just some thoughts. Not legal advice.
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Sneak up on them & check for a pulse...

Otherwise, stalk them on social networks to see if they are posting, tweeting, etc. Are they on gmail? Check to see if they are on gchat. Send them a certified letter or package that requires a signature and check the signature online. Make sure you label the package or letter to look like something they will accept.
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Go knock on their door.
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