You use a high voltage probe, these used to be fairly commonly available, and plugged in to the standard AVO 8 multi-meter. The AVO 8 was (and still is) an analogue multi-meter with a specification of 20,000 ohms/volt, so on the 1000V range had a resistance of 20,000,000 ohms. The probe was a simple series resistor - a VERY large one, in a thick plastic handle, with a probe at one end and a lead at the other that plugged in to the Avo. For a 30KV range the resistor needed to be 29 times the resistance of the meter, 580,000,000 ohms.
At FSD it would draw 50uA from the EHT, as the meter did on all ranges, notice that a digtal meter with it's 10Mohm resistance on all ranges would take 100uA on it's 1000V range!.
So for a 30KV range on a 1000V digital meter you require a series resistance of 290Mohms - bearing in mind the voltage ratings that resistors can handle!. Perhaps 29x10Mohm resistors in series?.