easiest way to tell...
 

...if carrera tensioners have been installed. Is there a way to tell without opening the engine (cam) ?

There is a very easy way to tell. If there is a tiny little oil line that comes off of the oil line to the cam towers and connects to the face plate of the chain boxes then you have carrera tensioners in your car. Here is a picture of the installation on the right side of the motor (credit Pelican Technical Article).
http://forums.pelicanparts.com/uploa...1254349097.jpg

You can see the oil line better in this photo (courtesy Jaybird's rebuild thread)

http://forums.pelicanparts.com/uploa...1254349647.jpg