If there are no outward tells then you have to know the player. It's pretty tough if you have never play this person before. But I have try and put the story together and see if there betting pattern makes sense.
only expirience make you know how =) on tournament notes on opponents can help you with. its really hard, especially if opponent not a newbie and can mask his hand, for example in deep stacked tournaments most of loose players unreadable.