How to tell similar Radicals apart?


For example the top of these characters:  
今 -> 人 (ひと)  
全 -> 入 (はいる)  
  
Or the side of these:  
明 -> 月 (つき)  
腹 -> 肉 (にく)  
  
Are there more examples like these?  
How do I know which is which?




> Are there more examples like these?

Another example of a very similar pair is 阝 (おおざと) and 阝 (こざとへん). In this case
the shapes are the same and the big difference is basically that the "hen" one
appears on the left and the other one appears on the right of the kanji. Yet
another one is 匚 (はこがまえ) and 匸 (かくしがまえ), which are distinguished only by a
small shape difference at the top left.

> How do I know which is which?

The good news is that the people who make kanji dictionaries are aware of
these problems and usually index things so that you can find kanji under
either one of the radicals. Perhaps a useful test of which kanji dictionary to
use would be whether the dictionary guides you through this difficulty.

