Watching players practice is a luck of the draw thing
As soon as you get to Flushing Meadow locate the practice courts and familiarise yourself how to get to them easily... then you will spend most of your time there walking round and round the practice courts waiting to see your player practice. Usually practice sessions last 1 hour, and the players often sit and watch the courts waiting to go on, so you will soon realise who will be taking over the courts. There will not be a handy list of who is practicing where, as this is private info for the players,
Players usually practice twice a day. Roger often practices on an "accessible" court and is often seen signing autographs after his session. You will have to work hard to see him and get his autograph - if that is what you would like to do - but it is definitely not impossible.
Have a great time!