Ggirl...its actually quite hard. Because its not about light or dark in terms of skin.
A lot of people would assume Olive skin is warm but apparently its cool.
And a lot of people think really porceline skin would be cool but it can be warm.
Apparently one way is to look at the veins on your wrist. If they appear very blue, you are cool. If they appear green, you are warm (presumably because the warm tones in your skin are like adding "yellow" to the blue of the vein making them appear green)
I've been pondering it a lot myself recently having tried to "diagnose" myself into a "season" (Seasonal Colour analysis - when people talk about getting their colours done etc). I'd thought I was cool but I think I am warm. I've been living as a "cool" person all these years too! I thought Pink blush looked better on me but I'm now thinking its because I've been wearing the wrong colours. I've spent far too much time thinking about it over the past couple of weeks that I've booked myself in to get draped not this weekend but the next!
But in a nutshell, if you are peaches and cream or peach blusher looks fab but cool pink doesn't, or for that matter if lilac eyeshadow doesn't do for you what everyone says it will, that's a good sign. If you look great in terracotta and olive but shit in lavender that's another good sign.
But it really is hard to objectively look at yourself and manage to deliberately not see other things that can confuse you like your hair colour (which might be dyed too) and eyes and contrasts and what you are wearing and fake tan and all that malarky...