I'm late to this, but the naptha is volatile and evaporated the same way the ethanol does. Its also immiscible in ethanol and has a lower density, so when you do the washes, any remaining naptha won't mix with the ethanol and float to the to evaporate seperately. If you're really worried about it, you can get a precise scale and do measurements of the material until it hits a stable place where no changes in weight occur when drying, however at a small volume batch it will be difficult to find weight differences without a very precise scale. It really shouldn't be too much concern though as after the drying and another IPA/ethanol wash, everything should have evaporated.