If you have a CD player or DAC with a volume control you can do a little test to determine the performance of your preamp. listen to the digital component through the preamp and then disconnect it and hook it up directly to your amp, being careful to start with the volume turned down low. Go back and forth a couple times. Hopefully there won't be much, if any difference in sound.
If it sounds the same without the preamp in the chain then it is doing its job. It should be neutral and transparent.