Those seals scrape oil from the valve stem off to stop it from entering the combustion chamber to be burnt off. Often times, when valve seals go bad, people often wind up replacing a slew of things and never truly fix the problem because as a part it's overlooked more frequently than something like the head-gasket would be when the exhaust is giving off different colors and you smell oil.
If you have a solid enough engine and you aren't worried about any of the major and more common seals having become deteriorated and the valve seals are basically the only maintenance in the top end you haven't done seal-wise, then go ahead and do it, it'll only help your engine.
As far as trying to test your current seals, I suppose if you could compress any given valve and watch if oil is running down the valve stem you would be able to tell the good ones from bad, but other than that idea, I think since you have the head off you should go ahead and do a top end gasket set, which includes valve seals. Use Copper Spray on the gaskets, torque to spec, maybe get some ARP head studs if you don't have them yet and you'll be set to go, mang.