As Jim said they have to be removed to test them. You need to check free length and then you have to have a valve spring checker to check the actual rate. Every brand has a different advertised rate.
I personally sell and use
RPM/KPM valve springs and can get you the specs, just shoot me an email.
I can only offer for identification purposes the Yamaha springs have paint on the upper portion of the spring. All other manufactures do not have distinctive marking that I can see.
Depending on the age of the engine the valve springs might just be what is replaced as the older the spring is the softer it gets due to heat & duty cycles.
Randy - RPM