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: 
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.
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.