You could try playing a note on your guitar, and make sure you know what note it it, and then try to sing it. Once you get good at singing in unison with your guitar, try different intervals like a perfect fifth, or a major seventh above the note your playing. After that, try singing along to scales on your guitar, in unison or harmony. This'll really get your ears going. Infact, its something I've been meening to do but haven't gotten around to cause I havent gotten past all the easier ear training excersises I know. :)