Miles Davis Autograph

Greetings Everyone -

While in college in the late 80's and 90's I was able to obtain a Miles Davis autograph on a poster I took to a concert.  This is not your ordinary autograph, either, as Miles drew on it as well!  I would be happy to share the story behind it, but it can get rather long, so I will only post my reason for being here right now.

How can I find out a value on this?  Since I got it myself in person, how can it be authenticated?  I had it matted and framed some time ago, and it hangs in a prominent spot in my home. 

If needed, I can post a picture, but would have to be from my personal laptop at home as I am currently on my work computer.

The drawing is of a profile of what appears to be himself, "looking back" at his picture on the poster (a very young Miles in the picture).  He wrote "Miles", then asked "who's this to?" and wrote "Mike".  Then said "you're a dog" and wrote "Dog".  I always say Miles Davis gave me my nickname, MikeDog.

Any help would be appreciated!  Thank you!