i will also add how to buy a down jacket there are a few points to consider:
how thick it is - this is the most important factor, thicker means greater volume so more air can be trapped
fill power - this is a measuring system used to grade the quality of the down, without going into details it essentially means the higher the grading the more air per cubic unit the down holds
feather to down ratio - not usually important as most companies use down of around 90 : 10
boxing - where the seams are, the more seams the better the down is held in position but this compromises warmth as the jacket will be thin where it is seamed, some jackets have boxing system which eliminates this, for example box wall construction
those are the factors indicating warmth
also some down jackets have a dwr coating which is handy or you can just proof it yourself