The Key to finding diminished chords is to be familiar with minor thirds. Here's a flash tutorial that helps you learn minor 3rds.

To find minor thirds SKIP TWO KEYS

Remember, to find minor thirds SKIP TWO KEYS and you will land on the minor third, which is the third key. I don't like to say it's three keys away, because then people get confused. It is three keys away, but you don't count the key you started on. So if you are playing a minor third, you are actually creating a shape that includes four keys! The two that you are playing and two in between. This is confusing if you try to think of it as three keys away. It's hard to tell the difference between the third key (not including the starting key) and the fourth key (inclusive of the start key), which is the third. I find this sort of thinking very confusing, so it's best to simply focus on the difference between the keys, which is two keys

To find out more, check out http://www.patternpiano.com/