Basically I want to test just how powerful my system can be, but I don't want to damage the amp or any of the speakers. Is there a safe way to do it?

Best test is to find the distortion point, by starting at lowest volume and slowly cranking it up to where it distorts. Remember the volume level displayed on the screen, and remember to not go past it with mates/chicks in the car :P

Basically I want to test just how powerful my system can be, but I don't want to damage the amp or any of the speakers. Is there a safe way to do it?

Ensuring the gains are set correctly, you are getting no clipping (when the system is being pushed so far that the power supply eg: amp or headunit can not provide anymore clean power) and listening for any other audible signs of distortion will ensure that you have less chance of blowing something.

If you are putting it on a meter (eg: termlab) to get a Decibel reading and you wanna just focus on the power from the subs, put a high pass on the speakers so they aren't trying to hit so hard.

Have your system set with an oscilloscope, for clean signals in, and check for beautifully curved sinewave on the power out, and check the output (volts) with white/pink noise (whatever they use) to set desired RMS you want to acheive.
Aim for 10% or more below maximum for equipment life. (often what is considered maximum for the connected equipment varies greatly from speaker to speaker)( based on research or experience - uhoh, there is that experience word again)
You can hear distortion @ 10% - damage starts @ 1%....
Stressing your system = $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

Basically I want to test just how powerful my system can be, but I don't want to damage the amp or any of the speakers. Is there a safe way to do it?

You could bench test it, out of the car.
Replace speakers with resistive loads, then measure current and voltage across all outputs.
Best to have an oscilloscope on the output, and a clean variable voltage signal to feed in. Crank it up until you get output signal distortion.
Do some simple maths.
Is this safe? Probably not, but it depends on the quality of your audio gear.

Alternatively, you could look at your amp and just believe the quoted outputs. lol