maybe not

I'm not quite sure you can. That is why many puchase title insurance during a pre-purchase.
The feds require a bill of sale, but that does not prove a lack of an existing lien on the plane itself.
 
First look up the airplane on http://registry.faa.gov/aircraftinquiry/NNum_Inquiry.aspx
There will be a link, just under the airworthiness block to order a CD with all the aircraft records for $10. Includes liens.
Takes about a week.
Or pay Plane Fax do it for you and you get it in one day or less.

I think that only includes liens that have been filed with the FAA. There is no legal requirement to do so, so you're not guaranteed to find all liens this way. It's a good start however.