Several important observations can be made from Figure 1.0:
1) The 1st, 4th, 5th and 8th intervals are all perfect
2) The 2nd, 3rd, 6th and 7th intervals are all major
These observations will hold true for any major scale and are the key to naming intervals!
Okay, so we understand what “major” and “perfect” intervals are, but what about the others: “minor”, “diminished” and “augmented”? The remaining three interval types can be thought of as deriving from the first two. The following table illustrates how we get “minor”, “diminished” and “augmented” intervals from “major” and “perfect” intervals:

Figure 2.0
And voila – you are now armed with all the tools you need to identify any interval!
Next, let’s work through a few exercises to check your understanding.