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: 
What OP means is that if you use the notes of that scale (no sharps or flats added or removed) and play a third and a fifth on each note you get those chords.

**Example on C major scale:**

C (first note):  

3rd up = E  

5th up = G  

Chord = C major 

D (second note):  

3rd up = F  

5th up = A  

Chord = D minor (note the half step between E/F making the `minor third`)

G (fifth note):  

3rd up = B  

5th up = D  

Chord = G major

A (sixth note):  

3rd up = C  

5th up = E  

Chord = A minor (note the half step between B/C making the `minor third`)

B (seventh note):  

3rd up = D  

5th up = F  

Chord = B dim (note the half step between B/C making the `minor third` and also half step between E/F making the fifth not perfect like all others but `dim`)
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.