Very first sign that a horse isn't ok inside

Yes. It's like Zen. The more you try to explain it, the more difficult it becomes. Yet it's something we all know on some instinctive level. Some just listen to it more than others.

I'm afraid it is something you can only learn from experience and even then I've ridden horses that with no warning at all can by batting off in the opposite direction - but they are often the ones that 'pretend' spookiness in order to have an excuse to go home or avoid something they don't want to do
In all of mine (under saddle) I feel a slight hesitation and some lightening in the front end, tension in the neck muscles, ears too sharply pricked, deeper breathing and maybe snorting. The quarters can often feel more active than they should as they are preparing for a spin
All these things can happen in a split second so a fast reaction is vital especially in the UK where we ride on busy roads a lot so it is important to 'know' your horse

The answer to that totally, totally depends.
When you say 'a horse isn't ok' that in itself can mean a huge variety of different things. It could be pain, emotional pain, fear, resistance, stress, worry, boredom, tolerance, mentally shut down etc and the list goes on and on.
And each time a horse has a feeling of 'not being ok' the result will differ every single time based on the tolerance and temperament of that individual horse, the stimulus it's receiving (either from you and/or it's environment) and the past influences of that individual horse.

With al the horses I've ridden I've noticed bucking and shying as there last resort. The first sign with my horse is tightened muscles, then head and ears up, then tries to spin and run in the opposite directions