I have noticed strange things happen right before dragons attack. How do I know when it's Time to grab my bow?
In my case, 5 out of 5 times the ground was shaking for a few seconds as if earthquake was happening. (Maybe sign it landed, though I always saw him in the air right after that)
When inside a city or a village (3 out the 5 times) the guards and armed citizens were running in a frenzy too, shooting with bows on the dragon. Note that until you don't attack the dragon yourself and hit it for the first time, you won't be engaged in combat.
At other times when I just saw a dragon hovering in the sky but without the above things nothing ever happened, even if I spent 100 arrows trying to lure it down.
The Sudden quake of the surroundings is the wind force from the dragons wings, therefore being quite close to the floor and to you, if you feel it again, try to look at the dragon next time, you will see how close he is.
now on things to see or hear when he is coming close, Firstly his Roar, sometimes they only do it when they spot you, but other times i have heard him without him spotting me.
If that is not a good one, then maybe a shadow zooming across the landscape is a good hint that there is something big flying above you.
Another is when in a city or town, as already said by shadow wizard, the villagers will be looking up, getting worried, running about, and you will hear people talking loudly saying , so all this will give it away for you too.
Hope that helps