There is a way to estimate it based on basic engineering...
When designing an automotive power amplifier, a basic decision that has to be made is whether to power the final stage with battery voltage or whether to boost the voltage.
Nobody is going to put in an expensive voltage booster just to double the power, so all automotive power amplifiers cluster into two power ranges.
At the lower, unboosted range (which is pretty much anything OEM, low cost, or physically small), you can make a rough estimate of the actual power going to a speaker by doing this:
Using the following calculator (requires Javascript):
[1] Enter your actual battery voltage (for systems with one side of the speaker grounded or connected together) or twice your actual battery voltage (for systems that run two wires to each speaker and warn you not to ground either side). [push-pull vs single ended]. [2] Enter in the impedence of a single speaker in ohms [3] Calculate watts. [4] Multiply by 70% [DC to RMS] or a bit lower (no system is perfectly efficient). [5] Add together the power from each speaker.ALTERNATIVE METHOD:
[1] Measure the current draw or take a guess from the fuse size. Enter it into the calculator. [2] Enter your actual battery voltage into the calculator. [3] Calculate Watts. [4] Reduce by 10-30% (no system is perfectly efficient).So a 13.5v battery and an 8 ohm speaker comes out at 16 watts, so four speakers gives you a 64 watt or lower system. It may be advertised as being 120 watts per speaker, but it isn't.
You can tell if the amp has a boosted power supply inside because it's bigger, heavier, more expensive, and claims to put out 500 or more watts per speaker. In that case the alternative method above will still work, but you can no longer assume 13.5 volts for the first method.