I'm a assistant music instructor
Well, when playing keys period piano,organ, or keyboards moving from left to right of your keyboard always start at the lowest tone to the highest....Now my personal advice about how to figure out what key someone is palying in is all a part of training your ear to tell the notes apart...example the lowest note would be "C" so on a keyboard that has five octives the highest note would also be "C." But also, knowing the "Root" note the bass player plays in that song is a good way of finding out what key the person is playing in, but not everyone plays on the same root. for example ; some horn players that are very strict on reading music will not play the root note that the bass player would play they will play using another note within that "root's" chord to play their riffs.....I hope this helps.....