Well the most common way is to mock up the valve in the head minus the spring and use snap gages...This inherintly has issues which make it somewhat inaccurate if not careful....The most accurate way that I know of is using a special micrometer pictured here http://www.shopcranecams.com/valsprinheig.html
This measurement is much to critical to guess at though.....Hope this helps
Use a 1/4" bolt cut just slightly shorter that the installed height you need. Then, thread on two nuts using one as a jam nut and then set them such (using your caliper) that they create the exact height you need (1.800" is a common height on smallblocks). Then use it as a go/no-go gauge by placing it between the spring seat and the surface the spring rides on under the spring retainer. Just install the retainer and locks and pull them up till they set. Ad shims until you get what you need and you're good to go. I reccomend taking a file and filing the head of the bolt smooth before you do this. It's worked for me many times. ::
They other way is to buy a valvespring micrometer ( but they cost about $50).