Well, the way Yahoo! displays their players, e.g. D. Wright (NYM - 3B
) will tell you all you need to know for your league.
That bold part is what positions your player is eligible to play under Yahoo's standards. A player like Travis Hafner has "Util." next to Cle in his profile, meaning he is available to play Utility and only Utility. Meanwhile, a DH like Papi played enough games at 1B last year to quality to play 1B, IF, as well as Utility. Search your player in Yahoo's player database, and you'll see where each specific player is eligible to play.