When does World of warcraft calculate if a spell can crit?

[Artamir]

Does WoW determine if a spell can crit at the start of the cast, or when it lands? I am talking about simple if it can crit, not will it crit.
Not percent to crit. IF it is capable of criting.

For instance :
Priest flash heal critable
Priest flash heal with Surge of Light proc incapable of a critical

I am wondering is it determined if a spell will crit when it is cast or when it lands.

[Forweg]

you cant determine exactly when a spell will crit or not. Although you can improve your chances of getting one. If you go into your gear, inventory screen where u can see your items of clothing you can see stats such as melee, base stats and theres one called spell damage. click on that and you can see the % that your spell will hit. yo ucan get higher percentage if u get it with your gear.
lvl 80 warlock

[Hama]

When it lands.

But, your spell crit% will increase if you get more gear with crit rating.