You can look for a "recommended fuse rating". They usually expect you to use external fuses (a few inches before the amp), to protect it.
For the Soundstream cr4000.1 you mentioned, they list that on
SonicElectronix. It's 80 amp on a 4 ga wire.
Using fuses to determine the power of an amp is not too effective. Most manufacturers round off to the next reasonable size. It will give you a decent estimate of what the amp can do... but should not be used as an "end-of-discussion" decider.
For example,
My Audiopipe's recommended fuse rating is 100 amp and 4 ga, but to get rated power out of it (1600@1ohm), you really need a 150 amp fuse. I can attest that it draws more than 100 amps DC, when at full power. Not sure why they did this...
I used to think fuse ratings were enough, but especially with external fuses, it's not so easy to tell. Internal fuses are usually shorted, during a bench test anyways. Moral of the story, you can do it, but take any results with a grain of salt.
Edit: You should know this already if you have a DD Z1. But besides fusing and rms ratings... there's not a good way to tell.