I tell by their behaviour and body postureing in different situations. Normally dominant dogs are confident dogs and hey will approach new/strange situations with ears and tail up, chest foreward and simply very confident. When greeting and interacting with other dogs they display dominant behaviour such as placing their head or paw over the other dogs' back, getting licked on the muzzle by subordinate dogs, correcting any dog that tries to mount it, mounting other dogs or people, etc etc. In more extreme cases where the human has not established pack hierarchy within the home, the dog will not tolerate humans trying to make them do anything such as if the dog is on the couch and someone tries to move him, he may try to "correct" the human by growling and biting. If a dog or human tries to step over top or lean over top of a very dominant dog they may try to "correct" them. Commands are sometimes flat out ignored because they dont feel the human giving the command has earned the respect of them obeying. Once you develop a good strong understanding of dog behaviour and their body language, you will be able to assess a dog's dominance level within minutes of seeing him interact with other dogs. Its easier to see when the dog is uinteracting with other dogs than humans because sometimes the dogs view all humans as alpha but other dogs are lower than them, and in other cases the dog has been trained to respect and maintain mannerisms towards all humans (though not necessarily accept them as alpha). For example, OC has been trained to respect and maintain mannerisms towards people to an extent so him being dominant isnt very evident to most people. If someone grabs his collar to move him or nudges him while laying down to get out of the way he will respect it and move (for the most part) but sometimes he gets testy with them (especially if I'm not around) and rarely will he listen to more than 1 or 2 commands if he listens to any at all. He is domiannt over humans.