RG-6 is a generic term, and is best interpreted as "sort of like". The "slash" number is a type designation, and that suffix came from the original, now obsolete, military "slash sheets". So RG-6/U meant something specific as to construction and specifications. Thus using just RG-6 essentially means that the cable in question somehow deviates from the original /U (or /A, /B, etc.) specification in some manner. /U actually means "multiple uses".
Here is a link to some information on the numbering scheme:
http://www.bluejeanscable.com/articles/rg6.htm
See also this topic in Wikipedia for another explanation, then follow the "coaxial cable" link in that article to get to a specific listing of RG type cables. You'll note they are (almost) all /U...
http://en.wikipedia.org/wiki/RG-6
As Iceberg noted, quad shield is usually marked as such, and it is also designated as such, e.g., RG-6/UQ...
EDIT - I agree with Mike500 that RG-6/U (and ALL coax for that matter) is unbalanced, but that is not what the /U means.