I'm definitely no expert, but for me I think it's more about the color scheme than symbols (although if your map is black and white, you might want to use the snowflake symbol as @rdanhenry suggested).

I'm posting a few links to maybe give you some inspiration. None of these are my works, I just googled "fantasy map elements snow" and browsed through the images. Post your map and everyone might be able to provide better insight. You may just even texture the area with dots or cross hatch or something similar that is unique to only your snow areas and simply label it such in your key. Hope this helps.

https://vignette.wikia.nocookie.net/...20080913162004

https://i.pinimg.com/236x/83/02/b4/8...gs-rpg-map.jpg

https://pre00.deviantart.net/27be/th...ef-d7ft5bz.jpg

http://moziru.com/images/drawn-mountain-map-5.jpg