Depends what slot you have it in. Say you have it in slot 4, then try using IN #4 and then loop over INPUT X,Y,B and PRINT X, Y, B or something like that and see if you get the desired values.

Edit: I just had a try with an emulator, but couldn't get it to work. Might have more luck remembering the details on my real machine at home (with a couple of books to refer to!)