Currently, there is no way I know of to get the hunger value of a food item, nor is there a way to query its saturation modifier.
A current workaround is to hardcode all hunger and saturation values of all edible foods, but that does not scale well with modded food items.

To fix this, I would like to suggest the following two functions (names subject to change):

item_hunger(item)

Returns the hunger value of the given item, or null if it is not a food item.

item_saturation_mod(item)

Returns the saturation modifier of the given item, or null if it is not a food item.

(The return value null can be substituted with 0 if necessary).