When a pet is tested, the way to see if they have hidden traits is to look at their URL code. Alternately, search the pet in Advanced search; carried traits are highlighted green, visible traits are blue, and visibles with a hidden trait are purple. IT also gives the trait letters.

oh, handy, I do seem to have a pet that has all of them for an example


If you mean 'will they be passed on from' in breeding, hidden traits can be passed down regardless of whether a pet is tested or not.
You just can't see that a pet has hidden traits unless it's tested.