Our trucks are equipped with a suspension load gauge which measures the PSI inside the tractor's rear suspension. It isn't exact, but you can tell most of the time when you need to scale a heavyish load and when you're still legal, on that set of axels at least.
When my gauge reads just over 50 I'm at my limit, so as you can see in this pic, this particular load of beer I took from St Louis was rather heavy. Fortunately I was able to slide the tandems forward far enough to even out the load -- on my first try even!
