The only way to tell how good they are is to listen to them. The best way still is direct comparison in the same room and system. Magazine reviews and recommendations from other people are useful to try to narrow down the possibilities. Some preamps offer a lot of features, and some offer bare minimum. Remember that some of the best preamps are the ones with the minimalist designs.
Amplifiers can range from a few watts of power to a few thousand.
Some claim that every amp sounds the same as long as it is not clipping. Every piece of equipment in the signal chain has an impact on the sound. All amps have their own sound. So you need to audition amps just like you should audition any other piece of equipment. But if the first watt sounds like din, who cares if you have 99 or 999 more.