One gets a pretty good idea from the MSLP charts.
What I tend to find is that in light-wind conditions (such as today's widespread high pressure) the wind can vary from say 10kt tail to 10kt head, and none of that was forecast. So one needs to build in a margin for that, e.g. 10% extra fuel even if no headwind at all was forecast.
And it can be totally different in both speed and direction between say 2000ft and say FL100.
In better defined conditions, where you have a definite head or tailwind, it can also vary by a lot, but e.g. a 40kt tailwind at FL100 is not likely to turn into a headwind, and if it does then you may need to land for fuel.
This is why, for long trips, a
fuel totaliser linked to a GPS is so invaluable. You get a constantly updated ETA, and Landing FOB. It is one of the cheapest gadgets one can fit to a plane, and is one of the most useful for avoiding enroute doubt and enhancing safety. I would not fly anywhere for real without mine.