How do you check a varistor with a ohmeter?

you need a very fancy ohm meter

if you dont have lotsa $ perhaps an ammeter a voltmeter and a variable current source would give reliable answers

maybe a megger since the ohm scale is 6v and the megohm scales use 2500V 1000V 500Vbut this is a lott poorer answer since 1000 ohms is 0 on the megohm

gigohm or 100gigohm scale

maybe a hipot tester depending on the low voltage and current settings

this is very effective with mov's since they are usually concerned with higher voltages