Which dominoes are missing?
CJam, 11 bytes

{:$7Ym*:$^}


An unnamed block (function) with I/O as a list of pairs of integers.

Test it here.

Explanation

:$   e# Sort each pair in the input.
7Ym* e# Get all pairs with elements in range [0 .. 6] using a Cartesian product.
:$   e# Sort each pair.
^    e# Symmetric set-difference. This will remove all pairs that are in the input
     e# and also remove duplicates, because it's a set operation.