SlewOnly is a utility plugin to use with Voxengo SPAN, in Density Mode. That will give you the best ‘VU style’, sense-the-loudness-of-the-track, metering, and when you put one of these utilities in front of SPAN, it’s calibrated so equal levels will give you appropriate amounts of extreme highs. If the levels are out of whack, your balances are also out of whack. (it’s fine to have quieter output with SlewOnly: peak levels should not be hotter than the regular mix, though.)
SlewOnly is a very simple algorithm: apart from some volume boosting to calibrate it, all that happens is that each sample has the previous sample subtracted. This produces an outrageous but very primitive highpass, so you can also use it as an audio filter.
This algorithm is available in Ditherbox, for convenience. That way you can switch it in for mix checking, without having to load extra plugins.