An inexpensive digital one will work — got mine at Harbor Freight either on sale or w/ a coupon:
(battery life seems to be hit or miss and random as to whether one gets a good unit, or one which eats batteries)
see https://www.shapeoko.com/wiki/index.php/Tools#Optional_tools
This one is popular and well-regarded:
To be honest though, I usually grab my Mitutoyo: