I think the 'carbon string' type are quite high resistance (to cut down wireless interference)- but it doesn't really matter since the volts are high and the current is very low. HT leads are failed when the high volts starts to leak out to the outside! Watch the engine running in the dark - if you see any sparks - they are knackered.
On the 'carbon string' type resistance should be less than 5000 ohms per inch. Over that they are starting to break down internally which equates to mis-firing. If you're using the HT leads with proper wire in them, the resistance should be VERY low.