Mmm... Do you mean how to distinguish those languages without having no idea about them? If it is your doubt... well, let me try:
Chinese vs. Korean = Korean has circles in its writing. Chineses has lots of strokes.
Dutch vs. German = Dutch repeats vowels and uses more consonants together than German.
Portuguese vs. Spanish = Spanish uses the letter 'Ñ / ñ' (I think it's the only language that uses it)
and Portuguese uses 'ç' or '~' on the top of certain vowels.

I think I can distinguish Chinese and Korean (aswell as Chinese and Korean) but I think Korean and Japanese is harder. As chopinxenakis said, there are usually some endings I can make out, especially -nida in korean (what does that even mean?).

Dutch vs. German is quite easy to me since I know basic German. Portugese vs. Spanish is impossible in my opinion :)

I can distinquish the difference sets of languages. The hardest would be Portuguese & Spanish because I'm not exposed to Portuguese.

I agree with 'chopinxenakis', the distinction of those three languages are there. I believe that you have to understand the basic characteristics of language, then you would be able see and hear the difference.