how can i tell what key something is in, lets say a guitar piece, without the sheet music?
Chief’s Domain - http://www.seas.ucla.edu/~ravi
I’m not all that musical, but I can pick out a song in C D A G etc on guitar. You play along and see if you are in tune, play by ear. I used to have a record I used to tune a guitar to, when I had no other reference point.
Its called perfect pitch. Either you’ve got it- some claim it can be taught,or you havn’t. Whether its a major or minor key should be pretty clear, then as Sunbear says, find the root note and you’re probably there.
Music almost invariably ends on the home chord, that’s the key of the piece. Frequently, phrases within the piece will end and/or begin there as well. If you know your notes you can hunt around for the melody, or the chords till you find it. Be advised though, that recorded music generally isn’t in tune to an insturment, therefore, in you wind up in D flat, the piece is probably in C
Also know, if you should go ahead and buy the sheet, that it is not necessarily in the same key as the record.
Catrandom
Additionally, you may not want to sing it in the key it was written or recorded in.
That used to be the rule. Now you see more sheet music for guitar that’s right.
By the way, being able to tell the key is NOT absolute perfect pitch, only relative pitch.
I asked my husband, and he said practice. He can tell what key stuff is in, but he’s been playing guitar for over 30 years.
For what it’s worth, he said the dial tone on the phone is “almost an F.” 
Sunbear you are wrong. Perfect pitch is the ability to discern any specific note or chord from a mere hearing without any aid (i.e. reference chord/note) being played before. Relative pitch is similar but you must have a reference point to be able to discern the pitch (i.e. a relative starting point). If you are a guitar player, as time passes you can tell what chords are being played by 1) their voicings (for simple chordal music). For example, an e minor chord is typically voiced from bottom to top ebegbe whereas an c major chord is typically voiced cegce (it is easier to hear the difference in intervals). Sometimes you can tell what pitch is being played by 2) the overtone series. The easiest way to hear these are from the residual hum that is sounded but not played. If you have a good guitar, the instrument should have sympathetic vibrations both from the soundboard and the strings. Typically the soundboard produces a good hum on either e’s or a’s. If you know your guitar well that should take care of that. My good guitar has the residual tone of a 440. Almost all instruments have this effect with the flute having the least (i.e. it has the purest tone without much if any overtones). Discerning the notes/chords in this way is really relative pitch becuase you are using the residual sound as a basis of another.
HUGS!
Sqrl
Gasoline: As an accompaniement to cereal it made a refreshing change. Glen Baxter
If you have a chromatic tuner/pitchpipe or any instrument that is mostly in tune you can discern what key something is in. Although in some cases there could be arguments made for 2 or more simultaneous keys.
I love to go around and figure out the pitches of ordinary objects (when they are struck and caused to vibrate), but I have a lot of free time…
Sweet Basil
One thing that I didn’t see anyone else mention is that there are different common keys for guitar (commonly: E, A, G, D) and piano (commonly: C, G, D). You’ll also come to realize that different instruments (and artists) tend to use different chord progressions.
Another thing to keep in mind is that some guitar players use non-standard tuning (Townshend) or stomp boxes (Lifeson) that will make you hear a ninth (or seventh) where you know good and well that it’s not possible to play one!
Well, almost not possible. I’m not taking into account those folks with more than five fingers per hand.
To make a long post just a little bit longer, I’ll say that if it’s a guitar-based song, start on open strings to find the root of the dominant chord, and go from there.
John