I always use a belt tension guage -- got one in '82 when I bought my first tractor.
I use a water-proof pen to mark a reference line on the housing behind the belt. Makes it easy to check the deflection without fiddling around with a straight edge.
It's a good idea to check all the belts when you first get your mill, whether it's new or used. My last mill shredded an alternator belt after 20 hours -- turns out they installed a cover wrong and the alternator wouldn't slack off enough. I guess whoever was assembling the mill just reefed away on the alternator until it clear the cover.