Assuming you're doing this on the cheap and have no measuring devices, here's what I recommend. But, first I would say that I could tell in about 15 seconds on my bike. Rig the wiring so that the resistor is hanging out the side. This will make comparison changes easier. Then, start it up and ride relatively slow. Coast so the load is relieved from the engine, and then slowly roll on the throttle. With the tre, you should feel a split second gradual roll-on that leads into the main power kick-in. Without the tre, the power kick in should be noticably sharper without the little ease-in roll-on.
Once the main power has kicked in, the primary effect of the tre is over. Low rpm sputtering from there on will need to be resolved with a Power Commander.
Next way is to get one of those gear indicators. But, for that, you could just get a real tre from ivan's and be done with it.