Is there anyway to tell which is bad without taking the starter down?
Yes there is...
Dig out your non-led test light and find a 24 volt bulb for it, find a 24 volt test light, or rig up some wire and a light bulb. Clamp the clip at the post under the hood you would like to test, stuff the light through the windshield, and use the probe to ground at a data plate screw or similar while you're cranking.
When mine was acting silly it was VERY intermittent. (I know you said yours is consistent, perhaps stay with the test light until or unless the problem becomes intermittant). I used a small board with four lights inside the cab, I strang an ugly string of yellow wires to test for starter button power to the starter relay, solenoid power out of the starter relay, battery power at the starter solenoid stud, and cranking power coming out of the starter solenoid stud. Very effective. The relay was "clicking" but not making contact. At the times it acted up, the bulb indicating power from the switch to the relay was weak. PS magazine found my problem, you have to MASH the starter button, and that will not happen. Don't mash the button, and this will eventually happen... No new parts in my case, just a little operator training, but the lights were VERY effective.
I had less than twenty five bucks in that contraption, including alligator clips for all the wires for easy hookup. I have since modified it so that I have two "common" wires, thus I can look for all power, all ground, or two of each. It's at work, and each bulb has it's own color of wire. I use it more often than similar equipment that cost me ten times as much. No soldier B required to run the in-cab controls while you stick your hands in places they shouldn't be with an engine that probably won't, but MIGHT decide to crank again.
Or you could die!! If you die that would mean we have one less friend... so don't die!! O.K.!! 
