What I do is I take notice of the road signs that I drive past - the last one that I went past that indicated the speed limit is the one I take to be the limit for that stretch of road.
That system is subject to failure, though, in instances in which the speed limit sign is missing, damaged or obscured, and particularly dodgy when (say) moving from a stretch of 'national limit' dual carriageway (70mph usually, in a car) into a built up area in which the limit is 30mph and if the sign is not visible you'd be unaware of the change, however an experienced motorist would know by other indications, such as the distance separating the street lights, the road markings, and the general surroundings, especially if a residential area.
Ignorance is no exemption from the law though, so even if a road sign was missing you would still be liable for a fine, however a court appearance might be more lenient due to the mitigating factors.
I find it good practice to 'drive according to the prevailing conditions'. EG on a road with a 50mph limit it would be sensible to reduce your speed in extreme weather conditions, poor light, degraded road surface, etc etc