i guess you could look at how quickly the solution is nebulised. you know it should normally take several minutes (depends on the volume of course).

thus, if it takes much less than this then it is probably being nebulised too quickly because of too high a flow rate, and therefore won't really be effective because the particle size will be too high or too low (i don't know the specifics of this).

that's all my guesswork though based on my experience and things others have told me over the years - i don't know for sure. if you find out more, let me know.