Chris, Dale had posted in the forum how to make the test box, it's dirt simple, just a couple of components. Here's a description from his website.
Testing the TMCC/Legacy Track Output Signal
Truthfully, I found that a much more definitive test was possible using an oscilloscope. I find the TMCC command base puts out about a 3V to 3.5V peak to peak signal, and the Legacy command base should be in the 5V to 6V range. The problem with the measurement is it seems any change in the circuit or meter used can throw off the results.