Yes a oscilloscope would be the best way to tune an amp. However with most people not having access to them nor knowing how to use it.
You can either tune your amp with a volt meter or by ear. Doing it by ear you have to listen for distortion and than back your gain off slightly. I have been doing this for years with no problems.
Amp Settings and Gains
So great you got your new amp and are wondering what in the world are all these knobs and switches on it for? Well here is your answer. Gain or volume. Some even say it differently than that but the deal with this is, think of it as a adjustment to insure your not clipping the signal. When you are all done installing your system is it time to put a test disk in of a normal song or a test tone of 80HRZ go to your amp and turn the gain all the way down than go back to the head unit and turn the volume 1/4 to full volume. Now you go back to your amp and slowly start turning up your gain until you hear distortion than turn it back just a hair. This will insure that your speakers are not going to distort or clip.
Now those Low pass and High pass knobs and all. Well a normal setup consists of your fronts being components. "speakers with separate tweeters" So the crossover will take care of the filtering of all that so your front should stay on Full. If you are going with rear speakers I would turn on the High pass filter and dial it to around 80 or maybe a bit higher. What these filter do are basically insure that the speaker will not get frequency's that it can not reproduce. The Low pass filter is fro when running a sub woofer your cut off should be around 60 for your sub.
Car Audio 101