Clear tube can get pretty dirty because it allows for growth. Reason being is that being clear it allows light to come in and that reacts with the moisture to cause some growth and discoloration by no means can determine condition of coil.
No! The tube won't be clear long enough to do you any good! Also I've seen dirty coils put out more condensate than clean coils. How would anyone judge what is the proper amount of condensate at any given point of time?