You do realize that it is more than just some "trick"? You have to spend many years in music to truly understand what is going on. Practice makes perfect. You do realize there are all kinds of 7th chords? Not just your basics by 7b5, 7#11's, 9th chords(which are 7add9's), 7b9, etc?
Can you detect intervals? Triads? Qualities? Extensions?
Rather than focusing on some abstract idea to "detect" some type of chord your time is better spent learning music. Learning music automatically teaches you everything you want. There is no short cut. Learn songs, learn to sight read, learn to improvise, etc... You'll never find some magic trick. The more you listen the more you'll figure out stuff. I used to think like you and I wasted many years, once I actually spent time learning music(listening and such) then everything made sense and I could hear everything I wanted.
But because you think it doesn't work this way: Every musical group of notes has a "color". 7th chords each have their own color. Dom7 chords have a certain quality to them. You won't under understand this unless you learn triads, maj 7ths, minor chords, diminished, tensions etc. In fact, they all work together. You build up the picture progressively not by sequentially. That is, you probably can tell major and minor apart, that is easy. But add a 4th note you get many more combinations. Min Add 9th, Min add11, Maj add#11, Maj add6, Sus2/4, etc. All these are just different colors/sounds. What makes them each unique is the unique combination of notes... but you learn to tell them apart by contrasting them with other colors AND by knowing the names of what you are hearing. Learn to spell chords and learn to analyze chords in music so you can at least know what you are suppose to hear, and then listen(= actively listen, which really means listening to a lot of different music. Don't try to hear some magical thing inside chords that doesn't exist).
A dom 7th chord has a tension in it that sets it apart from all the other non dominant chords. Listen to blues, you'll hear dominants all over the place. Have you listened to much blues? BB King? SRV? Albert King? Blues is the sound of the "dominant". Of course almost every piece of music uses dominants. Learn structural form of music. E.g., the blues is obvious and simple. The V7 chord comes at the 9th bar almost ALWAYS. So you can hear what it sounds like there. Learn to count so you know where things are at then you will know.
You are simply not going to be able to figure it out through some abstract process of practicing to hear some type of chord out of context and meaning... it's simply not how things work. Even if you do figure it out do you just want to be a one trick pony? Don't try to rush things, just learn and listen and over time things will make sense.
Now, I'm definitely not saying don't try to do what you are doing, just spend about 5 minutes a year on it, spend the rest of the time being more productive. Once I stopped wasting my time on trying to figure out shortcuts(which is what these "ear training" stuff is) and started working on actual music, after about 2 years I could ear almost all the chords. The biggest thing is to actually learn songs. How many songs do you know that uses dominant 7th chords? If you don't know any songs then what is the point? If you know some songs then figure out the where the dominant 7th chords are and that is what they sound like. Every other chord will be the same. The IV, well, it's right there, do you see it? You don't! That is why you don't know it is a IV chord. Analyze music! All music is just chords, so it's pretty easy.
It really is very difficult on one hand but very simple on the other. You have to learn to see the simplicity of it. It mainly has to do with exposure both intellectually and aurally. Sure, you probably can learn to hear these chords by practicing in some way but it will take you 10 times longer and you will get 100 times less out of it and that is not how you want to approach it.
What is the V7b9/V in the key of D# major? Can you figure out the notes relatively quickly? What are the intervals of a F#13#11? What chord does a the notes A C Eb F# B want to resolve to? These are intellectual things that you learn by studying, it has nearly zero to do with sound. It is the mathematics of music. Listening to music is where you can apply these things. If you know a piece uses a V7b9/V because you've analyzed it then you can listen to it and say "Oh, that is how it sounds". You do the "Oh" part a few (hundred) thousand times then everything makes sense.... but it happens gradually and you'll never be completely finished.