Finding Intervals in Keys Other than C

The best way to find intervals in other keys is to have all the Major scales
memorized. There are only 12 of them, so it shouldn’t take too long.
Because I’m originally a trumpet player, I still figure out intervals by
running up the major scales with trumpet fingerings. It’s sort of like
counting on your fingers. Very handy.
Another way is to memorize how many whole and half steps are in each
interval. This takes a lot of brain space, but it’s possible. Here’s a little
table with the number of whole and half steps for each interval.