Just check the new growth nodes of the plant along the main stem. If they are alternating, they are sexually mature and will begin flowering on time:
..||
..||//
..||
\\||
..||
..||//
[ignore the '.' - just needed to preserve format]
If the nodes are putting out symmetrical growth, it is still pre-mature and won't start flowering yet:
..||
\\||//
..||
\\||//
..||
\\||//
I think most plants will become sexually mature after about a month...
So if your plants are a few months old they will be fine for going into flowering. You got a little bit of a late start but not really.
12" high and several months old seems a bit slow growing to me though (even for outdoors)... did you amend the soil they are grown in and have you been using any sort of nutrients?