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`)
In the major scale, the first and fourth degrees are major. The fifth degree is dominant (major 3rd but minor 7th). The second, third and sixth degrees are minor, and the seventh degree is a minor with a diminished fifth (also known as minor 7th flat 5 or something like that, I'm not too good with chord names in English).

So for example in the C major scale: C and F are major, G is dominant, D, E and A are minor, and B is the one with the diminished fifth.