You can use calipers. The cheapest way is get a 6" digital vernier from Harbor Freight. Usually around $10.00. They work well for this situation.
The best is a spring "Height mike" available from shop suppliers. Or even Summit.
Spring "Kits" for heads are hit/miss. Unless they are assembled for your specific heads. It is up to the builder to correct the specs.. GM heads use different spring pocket heights. For rotators on exhaust. Most universal kits don't allow for this.
Several times a year a customer will bring me heads that have aftermkt springs/retainers installed. Complete with a "miss". I check the springs/height and invariably there are no spring shims on the exhaust side. This results in about 30 lb. of spring pressure.