there is unfortunately no way to tell anything about the traffic of a domain without owning it (whether or not it is registered is irrelevant as well). What you could do, if you are asking because you want to purchase based off traffic, is the following:1) Register the domain with a registrar that provides you with a grace period (they all have the capability of doing this, but most fail to because they are money hungry scavengers).
2) Set the name servers appropriately.
3) Create or use an existing script to count the number of unique visits to the domain that do not have a http referrer.
4) Drop the domain within the 5 day grace period (if the traffic is not what you hoped for).
You can get a very rough idea by typing in the domain.com into the Overture keyword tool. It will always show less than the domain gets, but if it gets 0 then you know its of little value. I reg'ed a typo once that showed 74 on OVT and in fact it got about 150-200 a day.