I would think that the absolute shortest route to monitoring some signal would be to dredge ebay and find a two
channel compressor like the Alesis 3630 or a Behringer AutoCom or Composer. The Alesis model can still be easily had for $99 at all the chain stores.
A pair of Y cables would allow you to insert them on the L/R and run them bypassed to simply
monitor the levels. Or, I'm thinking you may have a
mixer that is small enough or old enough that it doesn't give you much metering, so you would want to patch directly through in that case.
I'd advise you to go off the shelf. While building a metering project may be fairly straight forward... really, what's your time worth. And is it worth your time to repair a
mixer if something happens to go wrong inside that Radio Shack experiment box and you fizzle your mix?
One further note, I'm starting to see Behringer DEQ2496 digital EQs turn up half dead on ebay. I had one go south on me a couple years ago and kept it around for a similar use. I would patch it in on the control room outs of my
console, that way whatever I solo-ed would turn up on the display and the L/R mix would be there otherwise. It's got three flavors of
level metering: Analog style VU, Peak/
RMS, and SPL if you have a calibrated mic (it's actually only a dB or two off if you use an SM-57 and calibrate the input properly). Another truly useful feature is the
RTA. You can watch the main mix or an aux buss and see exactly what frequency is feeding back. It's also useful during sound checks to get a quick visual of a signal that doesn't seem to want to get EQ'd to your satisfaction.