I just wrote this page for another thread (http://www.mmo-champion.com/class-dr...esto-pve-help-)/) but everyone might be interested in giving it a go: http://gcdtest.110mb.com/
The idea is to test people's ability to cast spells just after every GCD (basically suited to Resto druids, but I'm sure it's good for others with instant casts). You press Start, then every second it will change the background colour after which you press the Cast Spell button and it will calculate your average delay after 10 GCDs. If you click too early the background will change to red until the GCD is ready.
Personally I get an average of about 90% success rate with ~50-70ms delay.