I’m not sure if this is the right sub-forum for this question, but nonetheless:
How do you train you eat to hear what chord is being played?
Ear Training
You can do that to a certain extent, but I think it’s generally going to be
A) Something you are naturally good at (it’s kind of like having perfect pitch) or
B) Something that just comes with experience. I don’t always do it, but I sometimes recognize chords I hear on a CD. Sometimes I recognize them wrong too 
I do a fair amount of transposing from CDs to alternate keys, and it goes much quicker than it used to. I think repetition is a big factor. With all that said, if you are wanting to improve your ability playing with others in a jam, it might be worth some effort to learn to recognize some basic chord shapes on other instruments. On a related note, being familiar with the effects of a capo is a big help. Ferinstance, if I see someone playing a song in G capoed at 3, I can figure out that it’s in Bb.
Most times I can recognize a particular chord form on a guitar when I hear it, but not usually the key. For instance, I might be able to recognize an open C chord form, but have to fish around to figure out where the capo is.
Key is the hard one. Sometimes it can be done if you are very aware of the slight tone differences in a certain instrument.
In Bluegrass
As for chords…if you mean whether the progression goes to the I, IIm, IV or V and so on. This is relatively easy with repetitive listening and identification.
The great thing about thinking of the chords in the number system comes in handy here. No matter the key, you begin to perceive patterns in how chords sound and in what orders they regularly occur. From these patterns you can derive some of the rules on western chord patterns.
If you move into harder progressions like swing, jazz, Nashville, the rules get a bit more complex as do the chords.
hope this makes sense…
As people have noted it depends if your talking IDing notes and chords out of thin air or relative to another note or chord.
The thin air one is tough, but can be done with practice or if you have perfect pitch or people who can ID chords and notes like the rest of us do color. It is actually a curse, someone people who have this have trouble playing in different keys other than the original song, because to them it is like watching a movie where all the colors are backward (red is blue, etc) that said over time (20+ yrs) I have noticed when picking songs off the Bluegrass station that I can get the key/chord on the first try pretty often, but definitely not 100%.
The ear train as “relative to another chord/note” is what I think of as the classic ear training, and here it can be taught with a little less time. When I took an ear train course at a Guitar workshop, they basically would teach you an interval or progression and have you associate it with a popular tune example 1st to a 5th - opening notes to Star Wars, 5 to 1 Flintstones, 1 to minor 3rd - heartbreaker (Led Zeppelin) . Here is a link to others http://www.theory.mikesparksmusic.com/files/Download/The%20Intervals.pdf
Chord progressions the same thing I, IV, V pretty common, I, V, vi, IV - dont stop believing, Journey and countless others… here is an Australian comedy group showing that progression is like 100 songs- https://www.youtube.com/watch?v=oOlDewpCfZQ