The basic test is with ignition on, you should feel the ISV vibrating.
To check for air leaks, with the engine running squeeze the pipe coming form the air filter box to the ISV, it should cut the engine out, if it keeps running the engine has an air leak so its letting un-metered air into the engine.
Hopefully its just a bit gummed up, take the ISV off & give it a clean with carb cleaner or petrol.