In England & Wales, each local authority has "definitive maps" which show precisely where the rights of way are. If it's on the map it's a right of way, and if it's not, it isn't. The map can't be wrong - it Is The Law! OS maps are not strictly speaking definitive but the information is copied from the definitive map, so if there is a right of way shown, you can safely assume you are genuinely allowed to walk there.
Rights of way can't lapse through disuse in England and Wales, they can only be formally extinguished - which hardly ever happens. If the situation on the ground doesn't match the map, it almost always means the landowner hasn't maintained the right of way properly.
In Scotland the position is far more complicated: rights of way are created and destroyed "on the fly", as it were, depending on whether people do or don't use a particular route "as of right" over a 20 year period. As there are no definitive maps to copy from, and as there is often controversy over whether a particular route is or isn't a right of way, OS maps don't show rights of way in Scotland.
However, with the new open access laws (which I think I saw being discussed here quite recently), rights of way have become much less important in Scotland than they are in England and Wales.
Incidentally, and before anyone complains, I realise this forum isn't just for England & Wales and Scotland... but those are the only countries I know about!