Bag seed is a great way to start your gardening skills. It saves money and also for the most part insures that the seed will be female.

The reason you are told to not use bag seed is that a majority of bag seed is hermaphrodite or very close to being so. Little amounts of stress can cause these bag seed to go hermaphrodite because of weaker genetics. This is not something you want if you plan to mother the plant or clone from it.

When plants are feminized they usually start with a very strong plant genetically and force it into growing a few male flowers while stressing the plant as little as possible. Feminized seeds are usually made by breeders who know what they're doing and will be more reliable than the average "bag seed" people get.

There is no way to tell the genetics of a plant simply by looking at it. Some can tell if it's Sativa or Indica dominant but even then, without actually knowing the parents of the plant there is no actual way to know the genetics.

Hope this helps,
Bob.