There are a few methods. Some builders use solid lifters in one cylinder to get a sample. Some simply measure using the outside of the lifter body.
If you have 'the touch', you can determine preload on a new, empty lifter by feeling when the lifter plunger hits the circlip, at the top of the lifter. It's important that your preload is somewhere around .025"-.040". So, I shoot for thirty but I never go over forty thousandths. I use NO valve springs when measuring valve-to-piston clearance. Absolutely no math is used; simply rotate the crank slowly while 'testing' the valve up and down. You can use a feeler gauge at the valve tip for a true reading provided you did your preliminary cam and timing chain set assembly first.
I use this information to verify my cam is properly timed. Some cams and factory timing gears come advanced or retarded but they normally don't say. Mechanics simply line up the timing marks, button it up and get paid. I want to know exactly where my cam is timed. - Dave