In the absence of an o-scope and test tones, you can get your car audio amplifiers gains setup "really close" by ear.
1. choose program material you're very familiar with - something cleanly recorded
2. back all gains on all amps out to zero
3. ensure crossover (if being used on the amps) are set correctly (another DIY to come)
4. set all tone controls on radio to flat - turn off all boosts, gains, media expanders, etc.
5. play your chosen program material - set radio volume to approximately 75% of wide open (if volume goes to 100, set to 75, etc)
6. if you're running deck power up front and you're setting up a sub amp, set the volume to the maximum undistorted volume (this will vary wildly from setup to setup).
7. for subwoofers - slowly back the gain in, listening very closely to the sub. Don't worry about "volume" so much as "stress". Listen intently for signs of distress. As you back the gain in, you should get a clean increase in output. The point where this increase stops is likely also the point where the sub begins to stress. At this point, stop.
7.1 for speakers, do the same. But listen to those speakers...listen for stress. Listen for distortion. Any sign of trouble. At this point, stop.
8. turn the gain down just a touch
9. play a few more dynamic tracks - making sure that you don't get any signs of distress on any of them
There you have it - the "tune by ear" method.