One of the best tricks I learned from a former teacher was a quick trick to figuring out thirds and fifths. Memorize this pattern: C E G B D F A C. And backwards: C A F D B G E C.
Quick mnemonic for it: “FACE-GBD.” (You don’t have to start on C; it’s more a trick for interval recognition).
Notice the distance between each note is a third in that pattern. C to E is a major third, E to G is a minor third, etc. They’re all thirds, regardless of if they’re major/minor.
And what happens when we stack two thirds? We get the interval of a fifth. Thus, C to G is a fifth, E to B is a fifth, etc.
When doing it backwards, keep in mind the inverted nature of the pattern. You’re going down in thirds, not up. So, if you go down two steps (two thirds), you don’t get a fifth, you get a fourth, because it’s inverted (see inverted intervals).
You can also figure out 7ths: the 7th note relative to C is B, for example. Just stack three thirds to find it (C to E, E to G, and G to B).
This is just a really, really, REALLY handy trick to being able to quickly figure out what the notes of a chord are.
TL;DR: memorize the pattern C E G B D F A C, forwards and backwards. It’s notes a third a part, and it’ll make your life a lot easier.