As far as determining whether someone is using a capo or not, that probably isn't too important: what is more important is deciding whether YOU will need a capo to play along with the record. When you learn to play in all the keys that can be played without barre chords, you will probably have a sense of the kinds of bass patterns characteristic of each of these keys, and hearing these bass patterns in music played in other keys will likely indicate that the player "transposed" from one key to the other using the capo so he could use his first position patterns higher up the neck. Many players can duplicate the patterns up the neck by barring, but fretted notes are usually not as clear and resonant as unfretted ones (of course, often accompaniments are played with barred chords specifically because the player wants to mute the resonance so the accompaniment doesn't overwhelm the lead).

--seed