Measure high voltages with a multimeter
The safest way is to buy a high voltage probe that is compatible with your meter. 

I use a Fluke one that is rated at 40kVDC. Note carefully that this kind of probe is adequate for DC or mains-frequency. If you have higher frequency high voltage it is not necessarily safe. 

Also, if you are using it on mains voltage  make sure it is rated for the proper category for the situation or you may be at great personal danger from arc flash. Do not use an inadequately rated or unrated multimeter or probe in situations where high fault currents can occur.