There are many betas of games for game consoles. Rom Hustler
For example, there is the beta (prototype) of Duck Tales - http://romhustler.net/rom/nes/394/duck-tales

Even if this can be considered abandoneware (up to the N64, PSX, Saturn) since they wont make any money, this can be considered legal? (for the final releases)

Hmm, nice site, I think I have heard of it before. Anyway, I know that Nintendo gets mad at websites who attemp to give out free Nintendo games without their permission no matter how old or new it is.

How do i find out wich beta it is or what build?

Here are my findings on that Duck Tales prototype package:

• Prototype: has menu changes, level select changes, continue option, hamburgers instead of ice cream
• b1: don't see anything different from the final
• b2 - b3: doesn't load, could be emulation problem or a bad dump
• o1 - o3: seems to be same as retail
• t+fre: final, french?
• t+por1.0 cbt: portugese?
• t+swe1.0_generalcoolnes: swedish, but hacked
• t+swe2.0_thetranslator: swedish, but hacked
• t-por: hacked

Also prototype and final release have different music of second level.