For familiars, it is quite easy to find that they have your AC.
How can I find the AC of my Animal Companion?
An animal companion's AC is calculated the same way as a PC: 10 + Dex modifier + proficiency + item bonus. By default, an animal companion is trained in barding and unarmored defense, and uses the level of its PC for proficiency bonuses.
For example, a bear companion belonging to a level 1 druid and wearing no barding has AC 10 (base) + 2 (DEX modifier) + 3 (trained unarmored defence at level 1) = 15.
The same bear belonging to a level 8 druid with the Mature Companion and Incredible Companion (Savage) feats and wearing light barding has AC 10 (base) + 4 (DEX modifier) + 10 (trained barding at level 8) +1 (light barding) = 25.