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.
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.