Good stock will become jelly-like in consistency. Skim off fat when cool. Freeze in ziplock bags or use within a few days. If you don't use it, place back on stove and bring to boil. Simmer for 10 minutes or so and place back in fridge.
Once strained, stock can be further concentrated by reducing it. Do this prior to seasoning. Bring to a simmer and allow to reduce by half.
Freeze in ice cube trays if not using within a few days