No Behaviour IS Behaviour. What do I mean by that?
It's not uncommon for someone to say "he was okay" at the groomer....vet...with other dogs....with people....in a large crowd....etc.
Just because your dog is not displaying outward signs of discomfort such as growling, barking lunging, jumping, to name a few, does not mean he's handling the situation well.
A shutdown dog is not okay. A dog who is too afraid to show discomfort due to previous learning history or environmental influences or other reasons - will appear to be "okay".
It's important to look at the dog's body. That is likely to give you information on how the dog is feeling.
Is he still? Are his muscles relaxed? Is he panting? Is he blinking his eyes? Does he appear stiff? Does he appear loose? What do his ears look like? What does his mouth look like? Are there ridges? Or is it smooth?
There are many signals which will give you a more accurate accounting of how your dog is feeling in a particular situation than assuming he's okay because you don't notice outward, obvious signs.
You don't have to become an expert in reading body language but it's a good idea to learn a few common signals.
My website resources page has numerous links to information on body language including Silent Conversations, Stop The 77, DogGoneSafe.