Java (JDK), 61 bytes

(a,b)->b<0||b>29||a>b+2&&a>21||a>30?0:a>20&&(a>29||a>b+1)?2:1

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Returns 0 for invalid games, 1 for valid ongoing games, 2 for valid finished games. Takes the score ordered, with the higher score first.

##Ungolfed

(a,b) ->
               b < 0           // Invalid if either number is negative
            || b > 29          // Invalid if b over 29
            ||    a > b + 2    
               && a > 21       // Invalid if a should've already won
            || a > 30          // Invalid if a over 30
          ? 0                  // Return 0 if invalid
          :      a > 20        // Can only be a win if a has at least 20 pts
              && (   a > 29    // If a is 30, a has won
                  || a > b + 1 // Otherwise a needs a 2 pt lead
                 )
            ? 2                // Return 2 for a win
            : 1                // Return 1 otherwise