How to differentiate "radical plus 0 strokes" characters?
Chinese native speakers don't treat 田 由 甲 申 甴 电 as related radicals. It is true that those radicals look very similar to each other, but we don't go further than that. We don't have any terms for this phenomenon, because we don't think it is something that needs special attention/study.

Here are more examples:  刀 vs 力, 土 vs 士, 己 vs 已 vs 巳,  冂 vs 几, etc.

In English, there are some words spelling almost same, for example: discreet vs discrete, complement vs compliment, calendar vs calender, etc. Are there any special terms for this phenomenon in English? Do you English native speakers treat them with special attention?

By the way, your question reminds me a character game I played when I was a  child: form as many characters as you can by adding another stroke to the character 日. All the characters appearing in your question are part of the answer. There are some more, like 旦, 白, etc.



Edited my answer as suggested by NS.X.

I went too extreme saying "Chinese native speakers don't treat 田 由 甲 申 甴 电 as related radicals." I admit that it was not true. Yes, they are related radicals, but only in the sense that they look very similar to each other, except for 由 and 甴 as pointed out by Stan, that they are variant forms of each other. 

The way that those characters 田 由 甲 申 甴 电 are listed under 田 as "radical plus 0 strokes" only reflects what the author(s) / editor(s) thought was appropriate. Are there any fundamental rules behind that? I doubt it.

Alternatively, I believe it could've been done in another way:


list 由 as a new radical, and list the following characters under it: 甹, 畁, and 甴 as a variant form of 由.
list 甲 as a new radical, and list 畢 under it.
list 申 as a new radical, and list the following characters under it: 畅, 畃.
list 电 as a new radical, and list  under it.


See, characters can be classified in different ways. 

I believe the author(s) / editor(s) of Kangxi Dictionary put 由 甲 申 甴 电 in the group of 田 for the sake of simplicity.