Question: How do I know which notes of a scale may be a Major/Minor chord?

Take [this](https://music.stackexchange.com/questions/6177/finding-out-which-scales-contain-a-specific-chord) question as an example. The OP states that the C major scale contains the following chords: C, Dm, Em, F, G, Am, Bdim

How do I accomplish this?

Answer: 
This comes from walking up the scale and taking triplets of notes which are spaced every other note: CEG, DFA, EGB, ... These are called "triads".

![enter image description here](https://i.stack.imgur.com/XcBP5.png)

The majors and minors have a perfect fifth above their root note, for instance the Am chord's A-E interval.

The seventh degree chord, B, is diminished: it has a minor third (B-D) and a diminished fifth (B-F).

You can transpose this pattern into any key.
This may have already been answered, but I'll throw in my grain of salt. If you remember, every triad is made up of 3 tones. The root, the third, and the fifth. If you play a C on your piano, then play one third up, and then play a third above that, you have a C major chord, because C-E is an M3. 

Continue and you get an m3 in D-F, 

an m3 in E-G, 

an M3 in F-A, 

an M3 in G-B,

an m3 in A-C

and an m3 in B-D. 

The reason that vii is diminished is because b-d-f is TWO m3s stacked on each other. This works in **EVERY** major scale. M is Major, and m is minor.