I use one of these.http://www.amazon.com/Krikit-V-Belt-Tension-Gauge-91107/dp/B000AOEJOE YMMV
 > Date: Wed, 24 Aug 2011 07:13:39 -0700
> From: camping.elliott@GMAIL.COM
> Subject: V-Belt tightness -- how to determine?
> To: vanagon@GERRY.VANAGON.COM
> 
> Something I've never understood about tightening the belt. Instructions
> usually say something like:
> 
> "Be careful not to get the belts too tight. You should have
> slight (1/8" - 1/4") deflection in the belts after they are adjusted.
> This of course depends on the length of the belt."
> 
> <http://www.vanagonauts.com/printable.phtml?&catid=215>
> 
> How much pressure do you put behind your thumb when testing? I can
> deflect the belt anywhere from not much to a lot more by pressing harder!
> 
> --
> Jack "Rocket j Squirrel" Elliott
> Bend, Ore.
> 1984 Westfalia. A poor but proud people.
> 1971 "Ladybug"-brand utility trailer ca. 1972 from a defunct company in
> San Clemente, Calif., now repurposed as The Westrailia.
> 
> Sent from my kitchen.