Often they will say what weather they are suited for if you go to a site like LLBean or Columbia. If you can, go to a store that carries that coat and stick your arm in it. That's what I do. I see how it warms up and try to determine "okay, this has a cinch at the bottom that will keep the air out" or "hrm, this is so puffy that the kid won't be able to move/it might be dangerous in a car seat." Go with somewhere with a good return policy if you are unsure. In other words, skip the Totsy sale if you aren't sure you're going to like the coat. My DD has done well thus far in her Circo 3 in 1 coat. I buy a size up and she uses them for 2 years. One layer only can be OK for the carseat and 2 layers is good for play.
ETA: To save money, try going to a consignment store or buying a decent-looking coat from somewhere cold, like Shopko.com (which sells clothing in MN). Some coat is better than no coat and it may be easier for them to get mittens and shirts than a coat.