Understanding whether you are playing against a novice or an amateur player or a professional player is not important anywhere as much as it is in the game of Rummy.
A professional player often plays taking into account the cards of everyone on the table including the card deck and the open cards and is often able to estimate the exact hand that you hold as also the cards that are available in the Book.
Not only that, you can also take it for granted that if you are playing hand-to-hand against a professional player, the chances of you getting a card that you desire from the Open deck rests only the first few couple of rounds till he hasn’t been able to gauge the exact hand that you hold. About 3-4 rounds down, your chance of getting your desired card is mainly from the Closed Deck.
Of Course that is qualified by the fact that even the Professional seasoned rummy player also slips once in a while.
As opposed to that if you are playing against a player of mid to beginner level, such an opponent will tend to focus mainly on his own cards and the chances of his discarding a card that you desire is often higher than that of when you are playing against a seasoned rummy player.
Additionally, apart from the skill level of the opponent, it might also be a good cue to gauge the fatigue levels of the player. Rummy is a game of intense concentration and a tired and fatigued mind can often lead to unwise moves being played at the opponents end. It is only fair to take advantage of such a lapse in judgment or calculation of your opponent in the Game of the Mind that is Rummy.